diff --git a/app/server_helpers.go b/app/server_helpers.go
index 86e792504..aa2937066 100644
--- a/app/server_helpers.go
+++ b/app/server_helpers.go
@@ -21,6 +21,6 @@ func respondWith(w http.ResponseWriter, code int, response interface{}) {
 	w.WriteHeader(code)
 	encoder := codec.NewEncoder(w, &codec.JsonHandle{})
 	if err := encoder.Encode(response); err != nil {
-		log.Errorf("Error encdoing response: %v", err)
+		log.Errorf("Error encoding response: %v", err)
 	}
 }
diff --git a/probe/docker/container.go b/probe/docker/container.go
index 7cacce98f..c88288d7d 100644
--- a/probe/docker/container.go
+++ b/probe/docker/container.go
@@ -360,21 +360,27 @@ func (c *container) NetworkInfo(localAddrs []net.IP) report.Sets {
 }
 
 func (c *container) memoryUsageMetric(stats []docker.Stats) report.Metric {
-	result := report.MakeMetric()
-	for _, s := range stats {
-		result = result.Add(s.Read, float64(s.MemoryStats.Usage)).WithMax(float64(s.MemoryStats.Limit))
+	var max float64
+	samples := make([]report.Sample, len(stats))
+	for i, s := range stats {
+		samples[i].Timestamp = s.Read
+		samples[i].Value = float64(s.MemoryStats.Usage)
+		if float64(s.MemoryStats.Limit) > max {
+			max = float64(s.MemoryStats.Limit)
+		}
 	}
-	return result
+	return report.MakeMetric(samples).WithMax(max)
 }
 
 func (c *container) cpuPercentMetric(stats []docker.Stats) report.Metric {
-	result := report.MakeMetric()
 	if len(stats) < 2 {
-		return result
+		return report.MakeMetric(nil)
 	}
 
+	samples := make([]report.Sample, len(stats)-1)
+	var max float64
 	previous := stats[0]
-	for _, s := range stats[1:] {
+	for i, s := range stats[1:] {
 		// Copies from docker/api/client/stats.go#L205
 		cpuDelta := float64(s.CPUStats.CPUUsage.TotalUsage - previous.CPUStats.CPUUsage.TotalUsage)
 		systemDelta := float64(s.CPUStats.SystemCPUUsage - previous.CPUStats.SystemCPUUsage)
@@ -382,14 +388,15 @@ func (c *container) cpuPercentMetric(stats []docker.Stats) report.Metric {
 		if systemDelta > 0.0 && cpuDelta > 0.0 {
 			cpuPercent = (cpuDelta / systemDelta) * float64(len(s.CPUStats.CPUUsage.PercpuUsage)) * 100.0
 		}
-		result = result.Add(s.Read, cpuPercent)
+		samples[i].Timestamp = s.Read
+		samples[i].Value = cpuPercent
 		available := float64(len(s.CPUStats.CPUUsage.PercpuUsage)) * 100.0
-		if available >= result.Max {
-			result.Max = available
+		if available >= max {
+			max = available
 		}
 		previous = s
 	}
-	return result
+	return report.MakeMetric(samples).WithMax(max)
 }
 
 func (c *container) metrics() report.Metrics {
diff --git a/probe/docker/container_test.go b/probe/docker/container_test.go
index 3ed5f78d8..3d200a337 100644
--- a/probe/docker/container_test.go
+++ b/probe/docker/container_test.go
@@ -92,8 +92,8 @@ func TestContainer(t *testing.T) {
 				docker.RestartContainer, docker.StopContainer, docker.PauseContainer,
 				docker.AttachContainer, docker.ExecContainer,
 			).WithMetrics(report.Metrics{
-			"docker_cpu_total_usage": report.MakeMetric(),
-			"docker_memory_usage":    report.MakeMetric().Add(now, 12345).WithMax(45678),
+			"docker_cpu_total_usage": report.MakeMetric(nil),
+			"docker_memory_usage":    report.MakeSingletonMetric(now, 12345).WithMax(45678),
 		}).WithParents(report.EmptySets.
 			Add(report.ContainerImage, report.MakeStringSet(report.MakeContainerImageNodeID("baz"))),
 		)
diff --git a/probe/host/reporter.go b/probe/host/reporter.go
index e93538b5e..ce68a387d 100644
--- a/probe/host/reporter.go
+++ b/probe/host/reporter.go
@@ -125,9 +125,9 @@ func (r *Reporter) Report() (report.Report, error) {
 	now := mtime.Now()
 	metrics := GetLoad(now)
 	cpuUsage, max := GetCPUUsagePercent()
-	metrics[CPUUsage] = report.MakeMetric().Add(now, cpuUsage).WithMax(max)
+	metrics[CPUUsage] = report.MakeSingletonMetric(now, cpuUsage).WithMax(max)
 	memoryUsage, max := GetMemoryUsageBytes()
-	metrics[MemoryUsage] = report.MakeMetric().Add(now, memoryUsage).WithMax(max)
+	metrics[MemoryUsage] = report.MakeSingletonMetric(now, memoryUsage).WithMax(max)
 
 	rep.Host.AddNode(
 		report.MakeNodeWith(report.MakeHostNodeID(r.hostID), map[string]string{
diff --git a/probe/host/reporter_test.go b/probe/host/reporter_test.go
index 08cf2ffed..bd1d2d823 100644
--- a/probe/host/reporter_test.go
+++ b/probe/host/reporter_test.go
@@ -20,9 +20,9 @@ func TestReporter(t *testing.T) {
 		hostname  = "hostname"
 		timestamp = time.Now()
 		metrics   = report.Metrics{
-			host.Load1:       report.MakeMetric().Add(timestamp, 1.0),
-			host.CPUUsage:    report.MakeMetric().Add(timestamp, 30.0).WithMax(100.0),
-			host.MemoryUsage: report.MakeMetric().Add(timestamp, 60.0).WithMax(100.0),
+			host.Load1:       report.MakeSingletonMetric(timestamp, 1.0),
+			host.CPUUsage:    report.MakeSingletonMetric(timestamp, 30.0).WithMax(100.0),
+			host.MemoryUsage: report.MakeSingletonMetric(timestamp, 60.0).WithMax(100.0),
 		}
 		uptime      = "278h55m43s"
 		kernel      = "release version"
@@ -88,10 +88,10 @@ func TestReporter(t *testing.T) {
 
 	// Should have metrics
 	for key, want := range metrics {
-		wantSample := want.LastSample()
+		wantSample, _ := want.LastSample()
 		if metric, ok := node.Metrics[key]; !ok {
 			t.Errorf("Expected %s metric, but not found", key)
-		} else if sample := metric.LastSample(); sample == nil {
+		} else if sample, ok := metric.LastSample(); !ok {
 			t.Errorf("Expected %s metric to have a sample, but there were none", key)
 		} else if sample.Value != wantSample.Value {
 			t.Errorf("Expected %s metric sample %f, got %f", key, wantSample, sample.Value)
diff --git a/probe/host/system_darwin.go b/probe/host/system_darwin.go
index ba682f1d1..33b3db5b6 100644
--- a/probe/host/system_darwin.go
+++ b/probe/host/system_darwin.go
@@ -45,7 +45,7 @@ var GetLoad = func(now time.Time) report.Metrics {
 		return nil
 	}
 	return report.Metrics{
-		Load1: report.MakeMetric().Add(now, one),
+		Load1: report.MakeSingletonMetric(now, one),
 	}
 }
 
diff --git a/probe/host/system_linux.go b/probe/host/system_linux.go
index 02c0a231b..cde84a3f2 100644
--- a/probe/host/system_linux.go
+++ b/probe/host/system_linux.go
@@ -45,7 +45,7 @@ var GetLoad = func(now time.Time) report.Metrics {
 		return nil
 	}
 	return report.Metrics{
-		Load1: report.MakeMetric().Add(now, one),
+		Load1: report.MakeSingletonMetric(now, one),
 	}
 }
 
diff --git a/probe/probe_internal_test.go b/probe/probe_internal_test.go
index 7c92451c2..55cd52e68 100644
--- a/probe/probe_internal_test.go
+++ b/probe/probe_internal_test.go
@@ -69,9 +69,6 @@ func (m mockPublisher) Stop() {
 }
 
 func TestProbe(t *testing.T) {
-	// marshalling->unmarshaling is not idempotent due to `json:"omitempty"`
-	// tags, transforming empty slices into nils. So, we make DeepEqual
-	// happy by setting empty `json:"omitempty"` entries to nil
 	const probeID = "probeid"
 	now := time.Now()
 	mtime.NowForce(now)
@@ -79,8 +76,11 @@ func TestProbe(t *testing.T) {
 
 	want := report.MakeReport()
 	node := report.MakeNodeWith("a", map[string]string{"b": "c"})
-	node.Metrics = nil // omitempty
-	// omitempty
+
+	// marshalling->unmarshaling is not idempotent due to `json:"omitempty"`
+	// tags, transforming empty slices into nils. So, we make DeepEqual
+	// happy by setting empty `json:"omitempty"` entries to nil
+	node.Metrics = nil
 	want.Endpoint.Controls = nil
 	want.Process.Controls = nil
 	want.Container.Controls = nil
diff --git a/probe/process/reporter.go b/probe/process/reporter.go
index 9087fb150..4b7bfc400 100644
--- a/probe/process/reporter.go
+++ b/probe/process/reporter.go
@@ -99,11 +99,11 @@ func (r *Reporter) processTopology() (report.Topology, error) {
 
 		if deltaTotal > 0 {
 			cpuUsage := float64(p.Jiffies-prev.Jiffies) / float64(deltaTotal) * 100.
-			node = node.WithMetric(CPUUsage, report.MakeMetric().Add(now, cpuUsage).WithMax(maxCPU))
+			node = node.WithMetric(CPUUsage, report.MakeSingletonMetric(now, cpuUsage).WithMax(maxCPU))
 		}
 
-		node = node.WithMetric(MemoryUsage, report.MakeMetric().Add(now, float64(p.RSSBytes)).WithMax(float64(p.RSSBytesLimit)))
-		node = node.WithMetric(OpenFilesCount, report.MakeMetric().Add(now, float64(p.OpenFilesCount)).WithMax(float64(p.OpenFilesLimit)))
+		node = node.WithMetric(MemoryUsage, report.MakeSingletonMetric(now, float64(p.RSSBytes)).WithMax(float64(p.RSSBytesLimit)))
+		node = node.WithMetric(OpenFilesCount, report.MakeSingletonMetric(now, float64(p.OpenFilesCount)).WithMax(float64(p.OpenFilesLimit)))
 
 		t.AddNode(node)
 	})
diff --git a/probe/process/reporter_test.go b/probe/process/reporter_test.go
index 0e664bcad..aaf176844 100644
--- a/probe/process/reporter_test.go
+++ b/probe/process/reporter_test.go
@@ -62,7 +62,7 @@ func TestReporter(t *testing.T) {
 	}
 	if memoryUsage, ok := node.Metrics[process.MemoryUsage]; !ok {
 		t.Errorf("Expected memory usage metric, but not found")
-	} else if sample := memoryUsage.LastSample(); sample == nil {
+	} else if sample, ok := memoryUsage.LastSample(); !ok {
 		t.Errorf("Expected memory usage metric to have a sample, but there were none")
 	} else if sample.Value != 0. {
 		t.Errorf("Expected memory usage metric sample %f, got %f", 0., sample.Value)
diff --git a/render/detailed/metrics_test.go b/render/detailed/metrics_test.go
index c233ec4ee..22c12f2a6 100644
--- a/render/detailed/metrics_test.go
+++ b/render/detailed/metrics_test.go
@@ -117,7 +117,7 @@ func TestNodeMetrics(t *testing.T) {
 func TestMetricRowSummary(t *testing.T) {
 	var (
 		now    = time.Now()
-		metric = report.MakeMetric().Add(now, 1.234)
+		metric = report.MakeSingletonMetric(now, 1.234)
 		row    = report.MetricRow{
 			ID:       "id",
 			Format:   "format",
diff --git a/render/detailed/summary_test.go b/render/detailed/summary_test.go
index 5fe3de9f7..514416c26 100644
--- a/render/detailed/summary_test.go
+++ b/render/detailed/summary_test.go
@@ -47,10 +47,10 @@ func TestSummaries(t *testing.T) {
 	// It should summarize nodes' metrics
 	{
 		t1, t2 := mtime.Now().Add(-1*time.Minute), mtime.Now()
-		metric := report.MakeMetric().Add(t1, 1).Add(t2, 2)
+		metric := report.MakeMetric([]report.Sample{{t1, 1}, {t2, 2}})
 		input := fixture.Report.Copy()
 
-		input.Process.Nodes[fixture.ClientProcess1NodeID] = input.Process.Nodes[fixture.ClientProcess1NodeID].WithMetrics(report.Metrics{process.CPUUsage: metric})
+		input.Process.Nodes[fixture.ClientProcess1NodeID].Metrics[process.CPUUsage] = metric
 		have := detailed.Summaries(input, render.ProcessRenderer.Render(input, nil))
 
 		node, ok := have[fixture.ClientProcess1NodeID]
diff --git a/render/metrics_test.go b/render/metrics_test.go
index 8f0212cd0..ef5be6ffb 100644
--- a/render/metrics_test.go
+++ b/render/metrics_test.go
@@ -31,20 +31,20 @@ func TestPropagateSingleMetrics(t *testing.T) {
 					report.MakeNode("child1").
 						WithTopology(report.Container).
 						WithMetrics(report.Metrics{
-							"metric1": report.MakeMetric(),
+							"metric1": report.MakeMetric(nil),
 						}),
 				),
 			),
 			topology: report.Container,
 			output: report.Nodes{
 				"a": report.MakeNode("a").WithMetrics(report.Metrics{
-					"metric1": report.MakeMetric(),
+					"metric1": report.MakeMetric(nil),
 				}).WithChildren(
 					report.MakeNodeSet(
 						report.MakeNode("child1").
 							WithTopology(report.Container).
 							WithMetrics(report.Metrics{
-								"metric1": report.MakeMetric(),
+								"metric1": report.MakeMetric(nil),
 							}),
 					),
 				),
@@ -57,30 +57,30 @@ func TestPropagateSingleMetrics(t *testing.T) {
 					report.MakeNode("child1").
 						WithTopology(report.Container).
 						WithMetrics(report.Metrics{
-							"metric1": report.MakeMetric(),
+							"metric1": report.MakeMetric(nil),
 						}),
 					report.MakeNode("child2").
 						WithTopology("otherTopology").
 						WithMetrics(report.Metrics{
-							"metric2": report.MakeMetric(),
+							"metric2": report.MakeMetric(nil),
 						}),
 				),
 			),
 			topology: report.Container,
 			output: report.Nodes{
 				"a": report.MakeNode("a").WithMetrics(report.Metrics{
-					"metric1": report.MakeMetric(),
+					"metric1": report.MakeMetric(nil),
 				}).WithChildren(
 					report.MakeNodeSet(
 						report.MakeNode("child1").
 							WithTopology(report.Container).
 							WithMetrics(report.Metrics{
-								"metric1": report.MakeMetric(),
+								"metric1": report.MakeMetric(nil),
 							}),
 						report.MakeNode("child2").
 							WithTopology("otherTopology").
 							WithMetrics(report.Metrics{
-								"metric2": report.MakeMetric(),
+								"metric2": report.MakeMetric(nil),
 							}),
 					),
 				),
@@ -93,12 +93,12 @@ func TestPropagateSingleMetrics(t *testing.T) {
 					report.MakeNode("child1").
 						WithTopology(report.Container).
 						WithMetrics(report.Metrics{
-							"metric1": report.MakeMetric(),
+							"metric1": report.MakeMetric(nil),
 						}),
 					report.MakeNode("child2").
 						WithTopology(report.Container).
 						WithMetrics(report.Metrics{
-							"metric2": report.MakeMetric(),
+							"metric2": report.MakeMetric(nil),
 						}),
 				),
 			),
@@ -109,12 +109,12 @@ func TestPropagateSingleMetrics(t *testing.T) {
 						report.MakeNode("child1").
 							WithTopology(report.Container).
 							WithMetrics(report.Metrics{
-								"metric1": report.MakeMetric(),
+								"metric1": report.MakeMetric(nil),
 							}),
 						report.MakeNode("child2").
 							WithTopology(report.Container).
 							WithMetrics(report.Metrics{
-								"metric2": report.MakeMetric(),
+								"metric2": report.MakeMetric(nil),
 							}),
 					),
 				),
@@ -127,32 +127,32 @@ func TestPropagateSingleMetrics(t *testing.T) {
 					report.MakeNode("child1").
 						WithTopology(report.Container).
 						WithMetrics(report.Metrics{
-							"metric1": report.MakeMetric(),
+							"metric1": report.MakeMetric(nil),
 						}),
 					report.MakeNode("child2").
 						WithLatest(report.DoesNotMakeConnections, now, "").
 						WithTopology(report.Container).
 						WithMetrics(report.Metrics{
-							"metric2": report.MakeMetric(),
+							"metric2": report.MakeMetric(nil),
 						}),
 				),
 			),
 			topology: report.Container,
 			output: report.Nodes{
 				"a": report.MakeNode("a").WithMetrics(report.Metrics{
-					"metric1": report.MakeMetric(),
+					"metric1": report.MakeMetric(nil),
 				}).WithChildren(
 					report.MakeNodeSet(
 						report.MakeNode("child1").
 							WithTopology(report.Container).
 							WithMetrics(report.Metrics{
-								"metric1": report.MakeMetric(),
+								"metric1": report.MakeMetric(nil),
 							}),
 						report.MakeNode("child2").
 							WithLatest(report.DoesNotMakeConnections, now, "").
 							WithTopology(report.Container).
 							WithMetrics(report.Metrics{
-								"metric2": report.MakeMetric(),
+								"metric2": report.MakeMetric(nil),
 							}),
 					),
 				),
diff --git a/report/controls.go b/report/controls.go
index 0a3cf2626..9b0a89487 100644
--- a/report/controls.go
+++ b/report/controls.go
@@ -4,7 +4,6 @@ import (
 	"time"
 
 	"github.com/ugorji/go/codec"
-
 	"github.com/weaveworks/scope/common/mtime"
 )
 
@@ -50,7 +49,7 @@ func (cs Controls) AddControls(controls []Control) {
 }
 
 // NodeControls represent the individual controls that are valid for a given
-// node at a given point in time.  Its is immutable. A zero-value for Timestamp
+// node at a given point in time.  It's immutable. A zero-value for Timestamp
 // indicated this NodeControls is 'not set'.
 type NodeControls struct {
 	Timestamp time.Time
@@ -86,15 +85,17 @@ func (nc NodeControls) Add(ids ...string) NodeControls {
 	}
 }
 
-// WireNodeControls is the intermediate type for json encoding.
-type WireNodeControls struct {
+// WireNodeControls is the intermediate type for encoding/decoding.
+// Only needed for backwards compatibility with probes
+// (time.Time is encoded in binary in MsgPack)
+type wireNodeControls struct {
 	Timestamp string    `json:"timestamp,omitempty"`
 	Controls  StringSet `json:"controls,omitempty"`
 }
 
 // CodecEncodeSelf implements codec.Selfer
 func (nc *NodeControls) CodecEncodeSelf(encoder *codec.Encoder) {
-	encoder.Encode(WireNodeControls{
+	encoder.Encode(wireNodeControls{
 		Timestamp: renderTime(nc.Timestamp),
 		Controls:  nc.Controls,
 	})
@@ -102,7 +103,7 @@ func (nc *NodeControls) CodecEncodeSelf(encoder *codec.Encoder) {
 
 // CodecDecodeSelf implements codec.Selfer
 func (nc *NodeControls) CodecDecodeSelf(decoder *codec.Decoder) {
-	in := WireNodeControls{}
+	in := wireNodeControls{}
 	if err := decoder.Decode(&in); err != nil {
 		return
 	}
diff --git a/report/metric_row.go b/report/metric_row.go
index ad965175e..f92bdae1f 100644
--- a/report/metric_row.go
+++ b/report/metric_row.go
@@ -54,6 +54,8 @@ func (*MetricRow) UnmarshalJSON(b []byte) error {
 	panic("UnmarshalJSON shouldn't be used, use CodecDecodeSelf instead")
 }
 
+// Needed to flatten the fields for backwards compatibility with probes
+// (time.Time is encoded in binary in MsgPack)
 type wiredMetricRow struct {
 	ID       string   `json:"id"`
 	Label    string   `json:"label"`
diff --git a/report/metric_template.go b/report/metric_template.go
index 092bbba97..2c093aac0 100644
--- a/report/metric_template.go
+++ b/report/metric_template.go
@@ -28,7 +28,7 @@ func (t MetricTemplate) MetricRows(n Node) []MetricRow {
 		Priority: t.Priority,
 		Metric:   &metric,
 	}
-	if s := metric.LastSample(); s != nil {
+	if s, ok := metric.LastSample(); ok {
 		row.Value = toFixed(s.Value, 2)
 	}
 	return []MetricRow{row}
diff --git a/report/metrics.go b/report/metrics.go
index 8b66dcb6a..b6b8b7a73 100644
--- a/report/metrics.go
+++ b/report/metrics.go
@@ -1,13 +1,10 @@
 package report
 
 import (
-	"bytes"
-	"encoding/gob"
 	"math"
 	"time"
 
 	"github.com/ugorji/go/codec"
-	"github.com/weaveworks/ps"
 )
 
 // Metrics is a string->metric map.
@@ -24,14 +21,18 @@ func (m Metrics) Lookup(key string) (Metric, bool) {
 func (m Metrics) Merge(other Metrics) Metrics {
 	result := m.Copy()
 	for k, v := range other {
-		result[k] = result[k].Merge(v)
+		if rv, ok := result[k]; ok {
+			result[k] = rv.Merge(v)
+		} else {
+			result[k] = v
+		}
 	}
 	return result
 }
 
 // Copy returns a value copy of the sets map.
 func (m Metrics) Copy() Metrics {
-	result := Metrics{}
+	result := make(Metrics, len(m))
 	for k, v := range m {
 		result[k] = v
 	}
@@ -41,7 +42,7 @@ func (m Metrics) Copy() Metrics {
 // Metric is a list of timeseries data with some metadata. Clients must use the
 // Add method to add values.  Metrics are immutable.
 type Metric struct {
-	Samples     ps.List
+	Samples     []Sample
 	Min, Max    float64
 	First, Last time.Time
 }
@@ -52,28 +53,55 @@ type Sample struct {
 	Value     float64   `json:"value"`
 }
 
-var nilMetric = Metric{Samples: ps.NewList()}
-
-// MakeMetric makes a new Metric.
-func MakeMetric() Metric {
-	return nilMetric
-}
-
-// Copy returns a value copy of the Metric. Metric is immutable, so we can skip
-// this.
-func (m Metric) Copy() Metric {
-	return m
-}
-
-// WithFirst returns a fresh copy of m, with First set to t.
-func (m Metric) WithFirst(t time.Time) Metric {
+// MakeSingletonMetric makes a metric with a single value
+func MakeSingletonMetric(t time.Time, v float64) Metric {
 	return Metric{
-		Samples: m.Samples,
-		Max:     m.Max,
-		Min:     m.Min,
+		Samples: []Sample{{t, v}},
+		Min:     v,
+		Max:     v,
 		First:   t,
-		Last:    m.Last,
+		Last:    t,
 	}
+
+}
+
+// MakeMetric makes a new Metric from unique samples incrementally ordered in
+// time.
+func MakeMetric(samples []Sample) Metric {
+	if len(samples) < 1 {
+		return Metric{}
+	}
+
+	var (
+		min = samples[0].Value
+		max = samples[0].Value
+	)
+
+	for i := 1; i < len(samples); i++ {
+		if samples[i].Value < min {
+			min = samples[i].Value
+		} else if samples[i].Value > max {
+			max = samples[i].Value
+		}
+	}
+
+	return Metric{
+		Samples: samples,
+		Min:     min,
+		Max:     max,
+		First:   samples[0].Timestamp,
+		Last:    samples[len(samples)-1].Timestamp,
+	}
+}
+
+// Copy returns a copy of the Metric.
+func (m Metric) Copy() Metric {
+	c := m
+	if c.Samples != nil {
+		c.Samples = make([]Sample, len(m.Samples))
+		copy(c.Samples, m.Samples)
+	}
+	return c
 }
 
 // WithMax returns a fresh copy of m, with Max set to max
@@ -89,10 +117,7 @@ func (m Metric) WithMax(max float64) Metric {
 
 // Len returns the number of samples in the metric.
 func (m Metric) Len() int {
-	if m.Samples == nil {
-		return 0
-	}
-	return m.Samples.Size()
+	return len(m.Samples)
 }
 
 func first(t1, t2 time.Time) time.Time {
@@ -109,83 +134,66 @@ func last(t1, t2 time.Time) time.Time {
 	return t2
 }
 
-// concat returns a new list formed by adding each element of acc to curr
-// acc or curr can be nil.
-func concat(acc []interface{}, curr ps.List) ps.List {
-	if curr == nil {
-		curr = ps.NewList()
-	}
-	for i := len(acc) - 1; i >= 0; i-- {
-		curr = curr.Cons(acc[i])
-	}
-	return curr
-}
-
-// Add returns a new Metric with (t, v) added to its Samples. Add is the only
-// valid way to grow a Metric.
-func (m Metric) Add(t time.Time, v float64) Metric {
-	// Find the first element which is before you element, and insert
-	// your new element in the list.  NB we want to dedupe entries with
-	// equal timestamps.
-	// This should be O(1) to insert a latest element, and O(n) in general.
-	curr, acc := m.Samples, make([]interface{}, 0, m.Len()+1)
-	for {
-		if curr == nil || curr.IsNil() {
-			break
-		}
-
-		currSample := curr.Head().(Sample)
-		if currSample.Timestamp.Equal(t) {
-			curr = curr.Tail()
-			break
-		}
-		if currSample.Timestamp.Before(t) {
-			break
-		}
-
-		acc, curr = append(acc, curr.Head()), curr.Tail()
-	}
-	acc = append(acc, Sample{t, v})
-	curr = concat(acc, curr)
-
-	return Metric{
-		Samples: curr,
-		Max:     math.Max(m.Max, v),
-		Min:     math.Min(m.Min, v),
-		First:   first(m.First, t),
-		Last:    last(m.Last, t),
-	}
-}
-
 // Merge combines the two Metrics and returns a new result.
 func (m Metric) Merge(other Metric) Metric {
-	// Merge two lists of samples in O(n)
-	curr1, curr2, acc := m.Samples, other.Samples, make([]interface{}, 0, m.Len()+other.Len())
-	var newSamples ps.List
 
+	// Optimize the empty and non-overlapping case since they are very common
+	switch {
+	case len(m.Samples) == 0:
+		return other
+	case len(other.Samples) == 0:
+		return m
+	case other.First.After(m.Last):
+		samplesOut := make([]Sample, len(m.Samples)+len(other.Samples))
+		copy(samplesOut, m.Samples)
+		copy(samplesOut[len(m.Samples):], other.Samples)
+		return Metric{
+			Samples: samplesOut,
+			Max:     math.Max(m.Max, other.Max),
+			Min:     math.Min(m.Min, other.Min),
+			First:   m.First,
+			Last:    other.Last,
+		}
+	case m.First.After(other.Last):
+		samplesOut := make([]Sample, len(m.Samples)+len(other.Samples))
+		copy(samplesOut, other.Samples)
+		copy(samplesOut[len(other.Samples):], m.Samples)
+		return Metric{
+			Samples: samplesOut,
+			Max:     math.Max(m.Max, other.Max),
+			Min:     math.Min(m.Min, other.Min),
+			First:   other.First,
+			Last:    m.Last,
+		}
+	}
+
+	// Merge two lists of Samples in O(n)
+	samplesOut := make([]Sample, 0, len(m.Samples)+len(other.Samples))
+	mI, otherI := 0, 0
 	for {
-		if curr1 == nil || curr1.IsNil() {
-			newSamples = concat(acc, curr2)
+		if otherI >= len(other.Samples) {
+			samplesOut = append(samplesOut, m.Samples[mI:]...)
 			break
-		} else if curr2 == nil || curr2.IsNil() {
-			newSamples = concat(acc, curr1)
+		} else if mI >= len(m.Samples) {
+			samplesOut = append(samplesOut, other.Samples[otherI:]...)
 			break
 		}
 
-		s1 := curr1.Head().(Sample)
-		s2 := curr2.Head().(Sample)
-
-		if s1.Timestamp.Equal(s2.Timestamp) {
-			curr1, curr2, acc = curr1.Tail(), curr2.Tail(), append(acc, s1)
-		} else if s1.Timestamp.After(s2.Timestamp) {
-			curr1, acc = curr1.Tail(), append(acc, s1)
+		if m.Samples[mI].Timestamp.Equal(other.Samples[otherI].Timestamp) {
+			samplesOut = append(samplesOut, m.Samples[mI])
+			mI++
+			otherI++
+		} else if m.Samples[mI].Timestamp.Before(other.Samples[otherI].Timestamp) {
+			samplesOut = append(samplesOut, m.Samples[mI])
+			mI++
 		} else {
-			curr2, acc = curr2.Tail(), append(acc, s2)
+			samplesOut = append(samplesOut, other.Samples[otherI])
+			otherI++
 		}
 	}
 
 	return Metric{
-		Samples: newSamples,
+		Samples: samplesOut,
 		Max:     math.Max(m.Max, other.Max),
 		Min:     math.Min(m.Min, other.Min),
 		First:   first(m.First, other.First),
@@ -195,14 +203,14 @@ func (m Metric) Merge(other Metric) Metric {
 
 // Div returns a new copy of the metric, with each value divided by n.
 func (m Metric) Div(n float64) Metric {
-	curr, acc := m.Samples, ps.NewList()
-	for curr != nil && !curr.IsNil() {
-		s := curr.Head().(Sample)
-		curr, acc = curr.Tail(), acc.Cons(Sample{s.Timestamp, s.Value / n})
+	samplesOut := make([]Sample, len(m.Samples), len(m.Samples))
+
+	for i := range m.Samples {
+		samplesOut[i].Value = m.Samples[i].Value / n
+		samplesOut[i].Timestamp = m.Samples[i].Timestamp
 	}
-	acc = acc.Reverse()
 	return Metric{
-		Samples: acc,
+		Samples: samplesOut,
 		Max:     m.Max / n,
 		Min:     m.Min / n,
 		First:   m.First,
@@ -210,20 +218,20 @@ func (m Metric) Div(n float64) Metric {
 	}
 }
 
-// LastSample returns the last sample in the metric, or nil if there are no
-// samples.
-func (m Metric) LastSample() *Sample {
-	if m.Samples == nil || m.Samples.IsNil() {
-		return nil
+// LastSample obtains the last sample of the metric
+func (m Metric) LastSample() (Sample, bool) {
+	if m.Samples == nil {
+		return Sample{}, false
 	}
-	s := m.Samples.Head().(Sample)
-	return &s
+	return m.Samples[len(m.Samples)-1], true
 }
 
 // WireMetrics is the on-the-wire representation of Metrics.
+// Only needed for backwards compatibility with probes
+// (time.Time is encoded in binary in MsgPack)
 type WireMetrics struct {
-	Samples []Sample `json:"samples,omitempty"` // On the wire, samples are sorted oldest to newest,
-	Min     float64  `json:"min"`               // the opposite order to how we store them internally.
+	Samples []Sample `json:"samples,omitempty"`
+	Min     float64  `json:"min"`
 	Max     float64  `json:"max"`
 	First   string   `json:"first,omitempty"`
 	Last    string   `json:"last,omitempty"`
@@ -244,14 +252,8 @@ func parseTime(s string) time.Time {
 // ToIntermediate converts the metric to a representation suitable
 // for serialization.
 func (m Metric) ToIntermediate() WireMetrics {
-	samples := []Sample{}
-	if m.Samples != nil {
-		m.Samples.Reverse().ForEach(func(s interface{}) {
-			samples = append(samples, s.(Sample))
-		})
-	}
 	return WireMetrics{
-		Samples: samples,
+		Samples: m.Samples,
 		Max:     m.Max,
 		Min:     m.Min,
 		First:   renderTime(m.First),
@@ -262,12 +264,8 @@ func (m Metric) ToIntermediate() WireMetrics {
 // FromIntermediate obtains the metric from a representation suitable
 // for serialization.
 func (m WireMetrics) FromIntermediate() Metric {
-	samples := ps.NewList()
-	for _, s := range m.Samples {
-		samples = samples.Cons(s)
-	}
 	return Metric{
-		Samples: samples,
+		Samples: m.Samples,
 		Max:     m.Max,
 		Min:     m.Min,
 		First:   parseTime(m.First),
@@ -299,20 +297,3 @@ func (Metric) MarshalJSON() ([]byte, error) {
 func (*Metric) UnmarshalJSON(b []byte) error {
 	panic("UnmarshalJSON shouldn't be used, use CodecDecodeSelf instead")
 }
-
-// GobEncode implements gob.Marshaller
-func (m Metric) GobEncode() ([]byte, error) {
-	buf := bytes.Buffer{}
-	err := gob.NewEncoder(&buf).Encode(m.ToIntermediate())
-	return buf.Bytes(), err
-}
-
-// GobDecode implements gob.Unmarshaller
-func (m *Metric) GobDecode(input []byte) error {
-	in := WireMetrics{}
-	if err := gob.NewDecoder(bytes.NewBuffer(input)).Decode(&in); err != nil {
-		return err
-	}
-	*m = in.FromIntermediate()
-	return nil
-}
diff --git a/report/metrics_test.go b/report/metrics_test.go
index 72fad1da6..012c3c42d 100644
--- a/report/metrics_test.go
+++ b/report/metrics_test.go
@@ -19,17 +19,17 @@ func TestMetricsMerge(t *testing.T) {
 	t4 := time.Now().Add(3 * time.Minute)
 
 	metrics1 := report.Metrics{
-		"metric1": report.MakeMetric().Add(t1, 0.1).Add(t2, 0.2),
-		"metric2": report.MakeMetric().Add(t3, 0.3),
+		"metric1": report.MakeMetric([]report.Sample{{t1, 0.1}, {t2, 0.2}}),
+		"metric2": report.MakeSingletonMetric(t3, 0.3),
 	}
 	metrics2 := report.Metrics{
-		"metric2": report.MakeMetric().Add(t4, 0.4),
-		"metric3": report.MakeMetric().Add(t1, 0.1).Add(t2, 0.2),
+		"metric2": report.MakeSingletonMetric(t4, 0.4),
+		"metric3": report.MakeMetric([]report.Sample{{t1, 0.1}, {t2, 0.2}}),
 	}
 	want := report.Metrics{
-		"metric1": report.MakeMetric().Add(t1, 0.1).Add(t2, 0.2),
-		"metric2": report.MakeMetric().Add(t3, 0.3).Add(t4, 0.4),
-		"metric3": report.MakeMetric().Add(t1, 0.1).Add(t2, 0.2),
+		"metric1": report.MakeMetric([]report.Sample{{t1, 0.1}, {t2, 0.2}}),
+		"metric2": report.MakeMetric([]report.Sample{{t3, 0.3}, {t4, 0.4}}),
+		"metric3": report.MakeMetric([]report.Sample{{t1, 0.1}, {t2, 0.2}}),
 	}
 	have := metrics1.Merge(metrics2)
 	if !reflect.DeepEqual(want, have) {
@@ -40,7 +40,7 @@ func TestMetricsMerge(t *testing.T) {
 func TestMetricsCopy(t *testing.T) {
 	t1 := time.Now()
 	want := report.Metrics{
-		"metric1": report.MakeMetric().Add(t1, 0.1),
+		"metric1": report.MakeSingletonMetric(t1, 0.1),
 	}
 	delete(want.Copy(), "metric1") // Modify a copy
 	have := want.Copy()            // Check the original wasn't affected
@@ -65,66 +65,24 @@ func checkMetric(t *testing.T, metric report.Metric, first, last time.Time, min,
 }
 
 func TestMetricFirstLastMinMax(t *testing.T) {
-	metric := report.MakeMetric()
-	var zero time.Time
+
+	checkMetric(t, report.MakeMetric(nil), time.Time{}, time.Time{}, 0.0, 0.0)
+
 	t1 := time.Now()
 	t2 := time.Now().Add(1 * time.Minute)
+
+	metric1 := report.MakeMetric([]report.Sample{{t1, -0.1}, {t2, 0.2}})
+
+	checkMetric(t, metric1, t1, t2, -0.1, 0.2)
+	checkMetric(t, metric1.Merge(metric1), t1, t2, -0.1, 0.2)
+
 	t3 := time.Now().Add(2 * time.Minute)
 	t4 := time.Now().Add(3 * time.Minute)
-	other := report.MakeMetric()
-	other.Max = 5
-	other.Min = -5
-	other.First = t1.Add(-1 * time.Minute)
-	other.Last = t4.Add(1 * time.Minute)
+	metric2 := report.MakeMetric([]report.Sample{{t3, 0.31}, {t4, 0.4}})
 
-	tests := []struct {
-		f           func(report.Metric) report.Metric
-		first, last time.Time
-		min, max    float64
-	}{
-		{nil, zero, zero, 0, 0},
-		{func(m report.Metric) report.Metric { return m.Add(t2, 2) }, t2, t2, 0, 2},
-		{func(m report.Metric) report.Metric { return m.Add(t1, 1) }, t1, t2, 0, 2},
-		{func(m report.Metric) report.Metric { return m.Add(t3, -1) }, t1, t3, -1, 2},
-		{func(m report.Metric) report.Metric { return m.Add(t4, 3) }, t1, t4, -1, 3},
-		{func(m report.Metric) report.Metric { return m.Merge(other) }, t1.Add(-1 * time.Minute), t4.Add(1 * time.Minute), -5, 5},
-	}
-	for _, test := range tests {
-		oldFirst, oldLast, oldMin, oldMax := metric.First, metric.Last, metric.Min, metric.Max
-		oldMetric := metric
-		if test.f != nil {
-			metric = test.f(metric)
-		}
-
-		// Check it didn't modify the old one
-		checkMetric(t, oldMetric, oldFirst, oldLast, oldMin, oldMax)
-
-		// Check the new one is as expected
-		checkMetric(t, metric, test.first, test.last, test.min, test.max)
-	}
-}
-
-func TestMetricAdd(t *testing.T) {
-	s := []report.Sample{
-		{time.Now(), 0.1},
-		{time.Now().Add(1 * time.Minute), 0.2},
-		{time.Now().Add(2 * time.Minute), 0.3},
-	}
-
-	have := report.MakeMetric().
-		Add(s[0].Timestamp, s[0].Value).
-		Add(s[2].Timestamp, s[2].Value). // Keeps sorted
-		Add(s[1].Timestamp, s[1].Value).
-		Add(s[2].Timestamp, 0.5) // Overwrites duplicate timestamps
-
-	want := report.MakeMetric().
-		Add(s[0].Timestamp, s[0].Value).
-		Add(s[1].Timestamp, s[1].Value).
-		Add(s[2].Timestamp, 0.5)
-
-	if !reflect.DeepEqual(want, have) {
-		t.Errorf("diff: %s", test.Diff(want, have))
-	}
+	checkMetric(t, metric2, t3, t4, 0.31, 0.4)
+	checkMetric(t, metric1.Merge(metric2), t1, t4, -0.1, 0.4)
+	checkMetric(t, metric2.Merge(metric1), t1, t4, -0.1, 0.4)
 }
 
 func TestMetricMerge(t *testing.T) {
@@ -133,20 +91,12 @@ func TestMetricMerge(t *testing.T) {
 	t3 := time.Now().Add(2 * time.Minute)
 	t4 := time.Now().Add(3 * time.Minute)
 
-	metric1 := report.MakeMetric().
-		Add(t2, 0.2).
-		Add(t3, 0.31)
+	metric1 := report.MakeMetric([]report.Sample{{t2, 0.2}, {t3, 0.31}})
 
-	metric2 := report.MakeMetric().
-		Add(t1, -0.1).
-		Add(t3, 0.3).
-		Add(t4, 0.4)
+	metric2 := report.MakeMetric([]report.Sample{{t1, -0.1}, {t3, 0.3}, {t4, 0.4}})
+
+	want := report.MakeMetric([]report.Sample{{t1, -0.1}, {t2, 0.2}, {t3, 0.31}, {t4, 0.4}})
 
-	want := report.MakeMetric().
-		Add(t1, -0.1).
-		Add(t2, 0.2).
-		Add(t3, 0.31).
-		Add(t4, 0.4)
 	have := metric1.Merge(metric2)
 	if !reflect.DeepEqual(want, have) {
 		t.Errorf("diff: %s", test.Diff(want, have))
@@ -159,8 +109,8 @@ func TestMetricMerge(t *testing.T) {
 	if !metric1.Last.Equal(t3) {
 		t.Errorf("Expected metric1.Last == %q, but was: %q", t3, metric1.Last)
 	}
-	if metric1.Min != 0.0 {
-		t.Errorf("Expected metric1.Min == %f, but was: %f", 0.0, metric1.Min)
+	if metric1.Min != 0.2 {
+		t.Errorf("Expected metric1.Min == %f, but was: %f", 0.2, metric1.Min)
 	}
 	if metric1.Max != 0.31 {
 		t.Errorf("Expected metric1.Max == %f, but was: %f", 0.31, metric1.Max)
@@ -173,13 +123,13 @@ func TestMetricMerge(t *testing.T) {
 }
 
 func TestMetricCopy(t *testing.T) {
-	want := report.MakeMetric()
+	want := report.MakeMetric(nil)
 	have := want.Copy()
 	if !reflect.DeepEqual(want, have) {
 		t.Errorf("diff: %s", test.Diff(want, have))
 	}
 
-	want = report.MakeMetric().Add(time.Now(), 1)
+	want = report.MakeSingletonMetric(time.Now(), 1)
 	have = want.Copy()
 	if !reflect.DeepEqual(want, have) {
 		t.Errorf("diff: %s", test.Diff(want, have))
@@ -190,12 +140,8 @@ func TestMetricDiv(t *testing.T) {
 	t1 := time.Now()
 	t2 := time.Now().Add(1 * time.Minute)
 
-	want := report.MakeMetric().
-		Add(t1, -2).
-		Add(t2, 2)
-	beforeDiv := report.MakeMetric().
-		Add(t1, -2048).
-		Add(t2, 2048)
+	want := report.MakeMetric([]report.Sample{{t1, -2}, {t2, 2}})
+	beforeDiv := report.MakeMetric([]report.Sample{{t1, -2048}, {t2, 2048}})
 	have := beforeDiv.Div(1024)
 	if !reflect.DeepEqual(want, have) {
 		t.Errorf("diff: %s", test.Diff(want, have))
@@ -218,55 +164,39 @@ func TestMetricMarshalling(t *testing.T) {
 		{Timestamp: t4, Value: 0.4},
 	}
 
-	want := report.MakeMetric()
-	for _, sample := range wantSamples {
-		want = want.Add(sample.Timestamp, sample.Value)
-	}
+	want := report.MakeMetric(wantSamples)
 
-	// gob
-	{
-		gobs, err := want.GobEncode()
-		if err != nil {
+	for _, h := range []codec.Handle{
+		codec.Handle(&codec.MsgpackHandle{}),
+		codec.Handle(&codec.JsonHandle{}),
+	} {
+		buf := &bytes.Buffer{}
+		encoder := codec.NewEncoder(buf, h)
+		if err := encoder.Encode(want); err != nil {
 			t.Fatal(err)
 		}
+		bufCopy := bytes.NewBuffer(buf.Bytes())
+
+		decoder := codec.NewDecoder(buf, h)
 		var have report.Metric
-		have.GobDecode(gobs)
+		if err := decoder.Decode(&have); err != nil {
+			t.Fatal(err)
+		}
+
 		if !reflect.DeepEqual(want, have) {
 			t.Error(test.Diff(want, have))
 		}
-	}
 
-	// others
-	{
-
-		for _, h := range []codec.Handle{
-			codec.Handle(&codec.MsgpackHandle{}),
-			codec.Handle(&codec.JsonHandle{}),
-		} {
-			buf := &bytes.Buffer{}
-			encoder := codec.NewEncoder(buf, h)
-			want.CodecEncodeSelf(encoder)
-			bufCopy := bytes.NewBuffer(buf.Bytes())
-
-			decoder := codec.NewDecoder(buf, h)
-			var have report.Metric
-			have.CodecDecodeSelf(decoder)
-
-			if !reflect.DeepEqual(want, have) {
-				t.Error(test.Diff(want, have))
-			}
-
-			// extra check for samples
-			decoder = codec.NewDecoder(bufCopy, h)
-			var wire struct {
-				Samples []report.Sample `json:"samples"`
-			}
-			if err := decoder.Decode(&wire); err != nil {
-				t.Error(err)
-			}
-			if !reflect.DeepEqual(wantSamples, wire.Samples) {
-				t.Error(test.Diff(wantSamples, wire.Samples))
-			}
+		// extra check for samples
+		decoder = codec.NewDecoder(bufCopy, h)
+		var wire struct {
+			Samples []report.Sample `json:"samples"`
+		}
+		if err := decoder.Decode(&wire); err != nil {
+			t.Error(err)
+		}
+		if !reflect.DeepEqual(wantSamples, wire.Samples) {
+			t.Error(test.Diff(wantSamples, wire.Samples))
 		}
 	}
 
diff --git a/test/fixture/report_fixture.go b/test/fixture/report_fixture.go
index c02e1c76a..9296d0acc 100644
--- a/test/fixture/report_fixture.go
+++ b/test/fixture/report_fixture.go
@@ -101,22 +101,22 @@ var (
 	ServiceUID          = "service1234"
 	ServiceNodeID       = report.MakeServiceNodeID(ServiceUID)
 
-	ClientProcess1CPUMetric    = report.MakeMetric().Add(Now, 0.01).WithFirst(Now.Add(-1 * time.Second))
-	ClientProcess1MemoryMetric = report.MakeMetric().Add(Now, 0.02).WithFirst(Now.Add(-2 * time.Second))
+	ClientProcess1CPUMetric    = report.MakeSingletonMetric(Now.Add(-1*time.Second), 0.01)
+	ClientProcess1MemoryMetric = report.MakeSingletonMetric(Now.Add(-2*time.Second), 0.02)
 
-	ClientContainerCPUMetric    = report.MakeMetric().Add(Now, 0.03).WithFirst(Now.Add(-3 * time.Second))
-	ClientContainerMemoryMetric = report.MakeMetric().Add(Now, 0.04).WithFirst(Now.Add(-4 * time.Second))
+	ClientContainerCPUMetric    = report.MakeSingletonMetric(Now.Add(-3*time.Second), 0.03)
+	ClientContainerMemoryMetric = report.MakeSingletonMetric(Now.Add(-4*time.Second), 0.04)
 
-	ServerContainerCPUMetric    = report.MakeMetric().Add(Now, 0.05).WithFirst(Now.Add(-5 * time.Second))
-	ServerContainerMemoryMetric = report.MakeMetric().Add(Now, 0.06).WithFirst(Now.Add(-6 * time.Second))
+	ServerContainerCPUMetric    = report.MakeSingletonMetric(Now.Add(-5*time.Second), 0.05)
+	ServerContainerMemoryMetric = report.MakeSingletonMetric(Now.Add(-6*time.Second), 0.06)
 
-	ClientHostCPUMetric    = report.MakeMetric().Add(Now, 0.07).WithFirst(Now.Add(-7 * time.Second))
-	ClientHostMemoryMetric = report.MakeMetric().Add(Now, 0.08).WithFirst(Now.Add(-8 * time.Second))
-	ClientHostLoad1Metric  = report.MakeMetric().Add(Now, 0.09).WithFirst(Now.Add(-9 * time.Second))
+	ClientHostCPUMetric    = report.MakeSingletonMetric(Now.Add(-7*time.Second), 0.07)
+	ClientHostMemoryMetric = report.MakeSingletonMetric(Now.Add(-8*time.Second), 0.08)
+	ClientHostLoad1Metric  = report.MakeSingletonMetric(Now.Add(-9*time.Second), 0.09)
 
-	ServerHostCPUMetric    = report.MakeMetric().Add(Now, 0.12).WithFirst(Now.Add(-12 * time.Second))
-	ServerHostMemoryMetric = report.MakeMetric().Add(Now, 0.13).WithFirst(Now.Add(-13 * time.Second))
-	ServerHostLoad1Metric  = report.MakeMetric().Add(Now, 0.14).WithFirst(Now.Add(-14 * time.Second))
+	ServerHostCPUMetric    = report.MakeSingletonMetric(Now.Add(-12*time.Second), 0.12)
+	ServerHostMemoryMetric = report.MakeSingletonMetric(Now.Add(-13*time.Second), 0.13)
+	ServerHostLoad1Metric  = report.MakeSingletonMetric(Now.Add(-14*time.Second), 0.14)
 
 	Report = report.Report{
 		ID: "test-report",