diff --git a/render/render.go b/render/render.go
index ff4e57924..b72293278 100644
--- a/render/render.go
+++ b/render/render.go
@@ -170,7 +170,8 @@ type joinResults struct {
 func newJoinResults(inputNodes report.Nodes) joinResults {
 	nodes := make(report.Nodes, len(inputNodes))
 	for id, n := range inputNodes {
-		n.Adjacency = nil // result() assumes all nodes start with no adjacencies
+		n.Adjacency = nil              // result() assumes all nodes start with no adjacencies
+		n.Children = n.Children.Copy() // so we can do unsafe adds
 		nodes[id] = n
 	}
 	return joinResults{nodes: nodes, mapped: map[string]string{}, multi: map[string][]string{}}
@@ -191,7 +192,7 @@ func (ret *joinResults) addUnmappedChild(m report.Node, id string, topology stri
 	if !exists {
 		result = report.MakeNode(id).WithTopology(topology)
 	}
-	result.Children = result.Children.Add(m)
+	result.Children.UnsafeAdd(m)
 	if m.Topology != report.Endpoint { // optimisation: we never look at endpoint counts
 		result.Counters = result.Counters.Add(m.Topology, 1)
 	}
@@ -219,6 +220,7 @@ func (ret *joinResults) addChildAndChildren(m report.Node, id string, topology s
 func (ret *joinResults) passThrough(n report.Node) {
 	n.Adjacency = nil // result() assumes all nodes start with no adjacencies
 	ret.nodes[n.ID] = n
+	n.Children = n.Children.Copy() // so we can do unsafe adds
 	ret.mapChild(n.ID, n.ID)
 }
 
diff --git a/report/node_set.go b/report/node_set.go
index 5331b3996..eef713748 100644
--- a/report/node_set.go
+++ b/report/node_set.go
@@ -24,8 +24,24 @@ func MakeNodeSet(nodes ...Node) NodeSet {
 	return emptyNodeSet.Add(nodes...)
 }
 
-// Add adds the nodes to the NodeSet. Add is the only valid way to grow a
-// NodeSet. Add returns the NodeSet to enable chaining.
+// Copy returns a value copy of the Nodes.
+func (n NodeSet) Copy() NodeSet {
+	result := ps.NewMap()
+	n.ForEach(func(node Node) {
+		result = result.UnsafeMutableSet(node.ID, node)
+	})
+	return NodeSet{result}
+}
+
+// UnsafeAdd adds a node to the NodeSet. Only call this if n has one owner.
+func (n *NodeSet) UnsafeAdd(node Node) {
+	if n.psMap == nil {
+		n.psMap = ps.NewMap()
+	}
+	n.psMap = n.psMap.UnsafeMutableSet(node.ID, node)
+}
+
+// Add adds the nodes to the NodeSet, and returns the NodeSet to enable chaining.
 func (n NodeSet) Add(nodes ...Node) NodeSet {
 	if len(nodes) == 0 {
 		return n