diff --git a/app/collector.go b/app/collector.go
index e2afae4a48..41274d3c42 100644
--- a/app/collector.go
+++ b/app/collector.go
@@ -1,11 +1,9 @@
 package app
 
 import (
-	"fmt"
 	"sync"
 	"time"
 
-	"github.com/spaolacci/murmur3"
 	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/common/mtime"
@@ -39,6 +37,7 @@ type collector struct {
 	reports []timestampReport
 	window  time.Duration
 	cached  *report.Report
+	merger  merger
 	waitableCondition
 }
 
@@ -78,6 +77,7 @@ func NewCollector(window time.Duration) Collector {
 		waitableCondition: waitableCondition{
 			waiters: map[chan struct{}]struct{}{},
 		},
+		merger: newSmartMerger(),
 	}
 }
 
@@ -110,15 +110,11 @@ func (c *collector) Report(_ context.Context) (report.Report, error) {
 	}
 	c.reports = clean(c.reports, c.window)
 
-	rpt := report.MakeReport()
-	id := murmur3.New64()
+	rpts := []report.Report{}
 	for _, tr := range c.reports {
-		rpt = rpt.Merge(tr.report)
-		id.Write([]byte(tr.report.ID))
+		rpts = append(rpts, tr.report)
 	}
-	rpt.ID = fmt.Sprintf("%x", id.Sum64())
-	c.cached = &rpt
-	return rpt, nil
+	return c.merger.Merge(rpts), nil
 }
 
 type timestampReport struct {
diff --git a/app/merger.go b/app/merger.go
new file mode 100644
index 0000000000..a93eb444f8
--- /dev/null
+++ b/app/merger.go
@@ -0,0 +1,111 @@
+package app
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/bluele/gcache"
+	"github.com/spaolacci/murmur3"
+
+	"github.com/weaveworks/scope/report"
+)
+
+type merger interface {
+	Merge([]report.Report) report.Report
+}
+
+type dumbMerger struct{}
+
+func (dumbMerger) Merge(reports []report.Report) report.Report {
+	rpt := report.MakeReport()
+	id := murmur3.New64()
+	for _, r := range reports {
+		rpt = rpt.Merge(r)
+		id.Write([]byte(r.ID))
+	}
+	rpt.ID = fmt.Sprintf("%x", id.Sum64())
+	return rpt
+}
+
+type smartMerger struct {
+	cache gcache.Cache
+}
+
+func newSmartMerger() merger {
+	return &smartMerger{
+		cache: gcache.New(100).LRU().Build(),
+	}
+}
+
+type node struct {
+	id          string
+	rpt         report.Report
+	left, right *node
+}
+
+type byID []*node
+
+func (ns byID) Len() int           { return len(ns) }
+func (ns byID) Swap(i, j int)      { ns[i], ns[j] = ns[j], ns[i] }
+func (ns byID) Less(i, j int) bool { return ns[i].id < ns[j].id }
+
+func hash(_1, _2 string) string {
+	id := murmur3.New64()
+	id.Write([]byte(_1))
+	id.Write([]byte(_2))
+	return fmt.Sprintf("%x", id.Sum64())
+}
+
+func (s *smartMerger) Merge(reports []report.Report) report.Report {
+	// Idea is to cache pair-wise merged reports, forming a merge
+	// tree.  Merging a new report into this tree should be log(N).
+
+	// Start with a sorted list of leaves
+	nodes := []*node{}
+	for _, r := range reports {
+		nodes = append(nodes, &node{
+			id:  r.ID,
+			rpt: r,
+		})
+	}
+	sort.Sort(byID(nodes))
+
+	// Define how to merge two nodes together.  Cache it.
+	merge := func(left, right *node) *node {
+		id := hash(left.id, right.id)
+
+		if result, err := s.cache.Get(id); err == nil {
+			return result.(*node)
+		}
+
+		n := &node{
+			id:    id,
+			rpt:   report.MakeReport().Merge(left.rpt).Merge(right.rpt),
+			left:  left,
+			right: right,
+		}
+		s.cache.Set(id, n)
+		return n
+	}
+
+	// Define how to reduce n nodes to n/2 nodes.
+	pass := func(nodes []*node) []*node {
+		output := []*node{}
+		for i := 0; i < len(nodes)/2; i++ {
+			output = append(output, merge(nodes[i*2], nodes[i*2+1]))
+		}
+		if len(nodes)%2 > 0 {
+			output = append(output, nodes[len(nodes)-1])
+		}
+		return output
+	}
+
+	// Iterative reduce n nodes down to 1
+	for len(nodes) > 1 {
+		nodes = pass(nodes)
+	}
+	if len(nodes) == 0 {
+		return report.MakeReport()
+	}
+	return nodes[0].rpt
+}