diff --git a/app/benchmark_internal_test.go b/app/benchmark_internal_test.go
index afc61e63f..fa5944654 100644
--- a/app/benchmark_internal_test.go
+++ b/app/benchmark_internal_test.go
@@ -72,6 +72,15 @@ func BenchmarkReportMerge(b *testing.B) {
 	}
 }
 
+func BenchmarkReportFastMerge(b *testing.B) {
+	reports := upgradeReports(readReportFiles(b, *benchReportPath))
+	merger := NewFastMerger()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		merger.Merge(reports)
+	}
+}
+
 func getReport(b *testing.B) report.Report {
 	r := fixture.Report
 	if *benchReportPath != "" {
diff --git a/app/merger.go b/app/merger.go
index 94231baf8..ac5c936f3 100644
--- a/app/merger.go
+++ b/app/merger.go
@@ -60,3 +60,21 @@ func (smartMerger) Merge(reports []report.Report) report.Report {
 	}
 	return <-c
 }
+
+type fastMerger struct{}
+
+// NewFastMerger makes a Merger which merges together reports, mutating the one we are building up
+func NewFastMerger() Merger {
+	return fastMerger{}
+}
+
+func (fastMerger) Merge(reports []report.Report) report.Report {
+	rpt := report.MakeReport()
+	id := murmur3.New64()
+	for _, r := range reports {
+		rpt.UnsafeMerge(r)
+		id.Write([]byte(r.ID))
+	}
+	rpt.ID = fmt.Sprintf("%x", id.Sum64())
+	return rpt
+}
diff --git a/app/merger_test.go b/app/merger_test.go
index 5ec36c307..ec114eedb 100644
--- a/app/merger_test.go
+++ b/app/merger_test.go
@@ -52,6 +52,10 @@ func BenchmarkDumbMerger(b *testing.B) {
 	benchmarkMerger(b, app.MakeDumbMerger())
 }
 
+func BenchmarkFastMerger(b *testing.B) {
+	benchmarkMerger(b, app.NewFastMerger())
+}
+
 const numHosts = 15
 
 func benchmarkMerger(b *testing.B, merger app.Merger) {
diff --git a/report/report.go b/report/report.go
index a217550e9..d21551c50 100644
--- a/report/report.go
+++ b/report/report.go
@@ -305,16 +305,21 @@ func (r Report) Copy() Report {
 // original is not modified.
 func (r Report) Merge(other Report) Report {
 	newReport := r.Copy()
-	newReport.DNS = newReport.DNS.Merge(other.DNS)
-	newReport.Sampling = newReport.Sampling.Merge(other.Sampling)
-	newReport.Window = newReport.Window + other.Window
-	newReport.Plugins = newReport.Plugins.Merge(other.Plugins)
-	newReport.WalkPairedTopologies(&other, func(ourTopology, theirTopology *Topology) {
-		*ourTopology = ourTopology.Merge(*theirTopology)
-	})
+	newReport.UnsafeMerge(other)
 	return newReport
 }
 
+// UnsafeMerge merges another Report into the receiver. The original is modified.
+func (r *Report) UnsafeMerge(other Report) {
+	r.DNS = r.DNS.Merge(other.DNS)
+	r.Sampling = r.Sampling.Merge(other.Sampling)
+	r.Window = r.Window + other.Window
+	r.Plugins = r.Plugins.Merge(other.Plugins)
+	r.WalkPairedTopologies(&other, func(ourTopology, theirTopology *Topology) {
+		ourTopology.UnsafeMerge(*theirTopology)
+	})
+}
+
 // WalkTopologies iterates through the Topologies of the report,
 // potentially modifying them
 func (r *Report) WalkTopologies(f func(*Topology)) {
diff --git a/report/topology.go b/report/topology.go
index 83baa0f94..49c01678c 100644
--- a/report/topology.go
+++ b/report/topology.go
@@ -163,6 +163,21 @@ func (t Topology) Merge(other Topology) Topology {
 	}
 }
 
+// UnsafeMerge merges the other object into this one, modifying the original.
+func (t *Topology) UnsafeMerge(other Topology) {
+	if t.Shape == "" {
+		t.Shape = other.Shape
+	}
+	if t.Label == "" {
+		t.Label, t.LabelPlural = other.Label, other.LabelPlural
+	}
+	t.Nodes.UnsafeMerge(other.Nodes)
+	t.Controls = t.Controls.Merge(other.Controls)
+	t.MetadataTemplates = t.MetadataTemplates.Merge(other.MetadataTemplates)
+	t.MetricTemplates = t.MetricTemplates.Merge(other.MetricTemplates)
+	t.TableTemplates = t.TableTemplates.Merge(other.TableTemplates)
+}
+
 // Nodes is a collection of nodes in a topology. Keys are node IDs.
 // TODO(pb): type Topology map[string]Node
 type Nodes map[string]Node
@@ -186,14 +201,19 @@ func (n Nodes) Merge(other Nodes) Nodes {
 		return n
 	}
 	cp := n.Copy()
+	cp.UnsafeMerge(other)
+	return cp
+}
+
+// UnsafeMerge merges the other object into this one, modifying the original.
+func (n *Nodes) UnsafeMerge(other Nodes) {
 	for k, v := range other {
-		if n, ok := cp[k]; ok { // don't overwrite
-			cp[k] = v.Merge(n)
+		if existing, ok := (*n)[k]; ok { // don't overwrite
+			(*n)[k] = v.Merge(existing)
 		} else {
-			cp[k] = v
+			(*n)[k] = v
 		}
 	}
-	return cp
 }
 
 // Validate checks the topology for various inconsistencies.