diff --git a/app/benchmark_internal_test.go b/app/benchmark_internal_test.go
index 1c9e06436c..9164ffe064 100644
--- a/app/benchmark_internal_test.go
+++ b/app/benchmark_internal_test.go
@@ -2,6 +2,7 @@ package app
 
 import (
 	"flag"
+	"math/rand"
 	"net/http"
 	"net/url"
 	"os"
@@ -65,6 +66,9 @@ func BenchmarkReportUpgrade(b *testing.B) {
 
 func BenchmarkReportMerge(b *testing.B) {
 	reports := upgradeReports(readReportFiles(b, *benchReportPath))
+	rand.Shuffle(len(reports), func(i, j int) {
+		reports[i], reports[j] = reports[j], reports[i]
+	})
 	merger := NewFastMerger()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
diff --git a/extras/generate_latest_map b/extras/generate_latest_map
index 78d1f9f6fe..3ae35b3981 100755
--- a/extras/generate_latest_map
+++ b/extras/generate_latest_map
@@ -74,27 +74,54 @@ function generate_latest_map() {
         return len(m)
     }
 
-    // Merge produces a fresh ${latest_map_type} containing the keys from both inputs.
+    // Merge produces a ${latest_map_type} containing the keys from both inputs.
     // When both inputs contain the same key, the newer value is used.
+    // Tries to return one of its inputs, if that already holds the correct result.
     func (m ${latest_map_type}) Merge(n ${latest_map_type}) ${latest_map_type} {
         switch {
-        case m == nil:
+        case len(m) == 0:
             return n
-        case n == nil:
+        case len(n) == 0:
             return m
         }
-        l := len(m)
-        if len(n) > l {
-            l = len(n)
+        if len(n) > len(m) {
+            m, n = n, m //swap so m is always at least as long as n
+        } else if len(n) == len(m) && m[0].Timestamp.Before(n[0].Timestamp) {
+            // Optimise common case where we merge two nodes with the same contents
+            // sampled at different times.
+            m, n = n, m // swap equal-length arrays so first element of m is newer
         }
-        out := make([]${entry_type}, 0, l)
 
         i, j := 0, 0
+    loop:
         for i < len(m) {
             switch {
-            case j >= len(n) || m[i].key < n[j].key:
-                out = append(out, m[i])
+            case j >= len(n):
+                return m
+            case m[i].key == n[j].key:
+                if m[i].Timestamp.Before(n[j].Timestamp) {
+                    break loop
+                }
                 i++
+                j++
+            case m[i].key < n[j].key:
+                i++
+            default:
+                break loop
+            }
+        }
+        if i >= len(m) && j >= len(n) {
+            return m
+        }
+
+        out := make([]${entry_type}, i, len(m))
+        copy(out, m[:i])
+
+        for i < len(m) {
+            switch {
+            case j >= len(n):
+                out = append(out, m[i:]...)
+                return out
             case m[i].key == n[j].key:
                 if m[i].Timestamp.Before(n[j].Timestamp) {
                     out = append(out, n[j])
@@ -103,6 +130,9 @@ function generate_latest_map() {
                 }
                 i++
                 j++
+            case m[i].key < n[j].key:
+                out = append(out, m[i])
+                i++
             default:
                 out = append(out, n[j])
                 j++
diff --git a/report/dns.go b/report/dns.go
index a09ed0b449..d50667c643 100644
--- a/report/dns.go
+++ b/report/dns.go
@@ -27,10 +27,12 @@ func (r DNSRecords) Merge(other DNSRecords) DNSRecords {
 	cp := r.Copy()
 	for k, v := range other {
 		if v2, ok := cp[k]; ok {
-			cp[k] = DNSRecord{
-				Forward: v.Forward.Merge(v2.Forward),
-				Reverse: v.Reverse.Merge(v2.Reverse),
+			fMerged, fUnchanged := v.Forward.Merge(v2.Forward)
+			rMerged, rUnchanged := v.Reverse.Merge(v2.Reverse)
+			if fUnchanged && rUnchanged {
+				continue
 			}
+			cp[k] = DNSRecord{Forward: fMerged, Reverse: rMerged}
 		} else {
 			cp[k] = v
 		}
diff --git a/report/id_list.go b/report/id_list.go
index 259b1d3b7c..e74901e7a4 100644
--- a/report/id_list.go
+++ b/report/id_list.go
@@ -23,7 +23,8 @@ func (a IDList) Add(ids ...string) IDList {
 
 // Merge all elements from a and b into a new list
 func (a IDList) Merge(b IDList) IDList {
-	return IDList(StringSet(a).Merge(StringSet(b)))
+	merged, _ := StringSet(a).Merge(StringSet(b))
+	return IDList(merged)
 }
 
 // Contains returns true if id is in the list.
diff --git a/report/latest_map_generated.go b/report/latest_map_generated.go
index 2cad405b73..97ce723823 100644
--- a/report/latest_map_generated.go
+++ b/report/latest_map_generated.go
@@ -42,27 +42,54 @@ func (m StringLatestMap) Size() int {
 	return len(m)
 }
 
-// Merge produces a fresh StringLatestMap containing the keys from both inputs.
+// Merge produces a StringLatestMap containing the keys from both inputs.
 // When both inputs contain the same key, the newer value is used.
+// Tries to return one of its inputs, if that already holds the correct result.
 func (m StringLatestMap) Merge(n StringLatestMap) StringLatestMap {
 	switch {
-	case m == nil:
+	case len(m) == 0:
 		return n
-	case n == nil:
+	case len(n) == 0:
 		return m
 	}
-	l := len(m)
-	if len(n) > l {
-		l = len(n)
+	if len(n) > len(m) {
+		m, n = n, m //swap so m is always at least as long as n
+	} else if len(n) == len(m) && m[0].Timestamp.Before(n[0].Timestamp) {
+		// Optimise common case where we merge two nodes with the same contents
+		// sampled at different times.
+		m, n = n, m // swap equal-length arrays so first element of m is newer
 	}
-	out := make([]stringLatestEntry, 0, l)
 
 	i, j := 0, 0
+loop:
 	for i < len(m) {
 		switch {
-		case j >= len(n) || m[i].key < n[j].key:
-			out = append(out, m[i])
+		case j >= len(n):
+			return m
+		case m[i].key == n[j].key:
+			if m[i].Timestamp.Before(n[j].Timestamp) {
+				break loop
+			}
 			i++
+			j++
+		case m[i].key < n[j].key:
+			i++
+		default:
+			break loop
+		}
+	}
+	if i >= len(m) && j >= len(n) {
+		return m
+	}
+
+	out := make([]stringLatestEntry, i, len(m))
+	copy(out, m[:i])
+
+	for i < len(m) {
+		switch {
+		case j >= len(n):
+			out = append(out, m[i:]...)
+			return out
 		case m[i].key == n[j].key:
 			if m[i].Timestamp.Before(n[j].Timestamp) {
 				out = append(out, n[j])
@@ -71,6 +98,9 @@ func (m StringLatestMap) Merge(n StringLatestMap) StringLatestMap {
 			}
 			i++
 			j++
+		case m[i].key < n[j].key:
+			out = append(out, m[i])
+			i++
 		default:
 			out = append(out, n[j])
 			j++
@@ -264,27 +294,54 @@ func (m NodeControlDataLatestMap) Size() int {
 	return len(m)
 }
 
-// Merge produces a fresh NodeControlDataLatestMap containing the keys from both inputs.
+// Merge produces a NodeControlDataLatestMap containing the keys from both inputs.
 // When both inputs contain the same key, the newer value is used.
+// Tries to return one of its inputs, if that already holds the correct result.
 func (m NodeControlDataLatestMap) Merge(n NodeControlDataLatestMap) NodeControlDataLatestMap {
 	switch {
-	case m == nil:
+	case len(m) == 0:
 		return n
-	case n == nil:
+	case len(n) == 0:
 		return m
 	}
-	l := len(m)
-	if len(n) > l {
-		l = len(n)
+	if len(n) > len(m) {
+		m, n = n, m //swap so m is always at least as long as n
+	} else if len(n) == len(m) && m[0].Timestamp.Before(n[0].Timestamp) {
+		// Optimise common case where we merge two nodes with the same contents
+		// sampled at different times.
+		m, n = n, m // swap equal-length arrays so first element of m is newer
 	}
-	out := make([]nodeControlDataLatestEntry, 0, l)
 
 	i, j := 0, 0
+loop:
 	for i < len(m) {
 		switch {
-		case j >= len(n) || m[i].key < n[j].key:
-			out = append(out, m[i])
+		case j >= len(n):
+			return m
+		case m[i].key == n[j].key:
+			if m[i].Timestamp.Before(n[j].Timestamp) {
+				break loop
+			}
 			i++
+			j++
+		case m[i].key < n[j].key:
+			i++
+		default:
+			break loop
+		}
+	}
+	if i >= len(m) && j >= len(n) {
+		return m
+	}
+
+	out := make([]nodeControlDataLatestEntry, i, len(m))
+	copy(out, m[:i])
+
+	for i < len(m) {
+		switch {
+		case j >= len(n):
+			out = append(out, m[i:]...)
+			return out
 		case m[i].key == n[j].key:
 			if m[i].Timestamp.Before(n[j].Timestamp) {
 				out = append(out, n[j])
@@ -293,6 +350,9 @@ func (m NodeControlDataLatestMap) Merge(n NodeControlDataLatestMap) NodeControlD
 			}
 			i++
 			j++
+		case m[i].key < n[j].key:
+			out = append(out, m[i])
+			i++
 		default:
 			out = append(out, n[j])
 			j++
diff --git a/report/latest_map_internal_test.go b/report/latest_map_internal_test.go
index e20e54038c..70ca27a7d8 100644
--- a/report/latest_map_internal_test.go
+++ b/report/latest_map_internal_test.go
@@ -93,13 +93,6 @@ func TestLatestMapMerge(t *testing.T) {
 			want: MakeStringLatestMap().
 				Set("foo", now, "bar"),
 		},
-		"Empty b": {
-			a: MakeStringLatestMap().
-				Set("foo", now, "bar"),
-			b: MakeStringLatestMap(),
-			want: MakeStringLatestMap().
-				Set("foo", now, "bar"),
-		},
 		"Disjoint a & b": {
 			a: MakeStringLatestMap().
 				Set("foo", now, "bar"),
@@ -117,10 +110,26 @@ func TestLatestMapMerge(t *testing.T) {
 			want: MakeStringLatestMap().
 				Set("foo", now, "bar"),
 		},
+		"Longer": {
+			a: MakeStringLatestMap().
+				Set("PID", now, "23128").
+				Set("Name", now, "curl"),
+			b: MakeStringLatestMap().
+				Set("PID", then, "0").
+				Set("Name", now, "curl").
+				Set("Domain", now, "node-a.local"),
+			want: MakeStringLatestMap().
+				Set("PID", now, "23128").
+				Set("Name", now, "curl").
+				Set("Domain", now, "node-a.local"),
+		},
 	} {
 		if have := c.a.Merge(c.b); !reflect.DeepEqual(c.want, have) {
 			t.Errorf("%s:\n%s", name, test.Diff(c.want, have))
 		}
+		if have := c.b.Merge(c.a); !reflect.DeepEqual(c.want, have) {
+			t.Errorf("%s:\n%s", name, test.Diff(c.want, have))
+		}
 	}
 }
 
diff --git a/report/report.go b/report/report.go
index d21551c504..5c065d1867 100644
--- a/report/report.go
+++ b/report/report.go
@@ -531,13 +531,12 @@ func (r Report) upgradeDNSRecords() Report {
 		if ok && (foundS || foundR) {
 			// Add address and names to report-level map
 			if existing, found := dns[addr]; found {
-				// Optimise the expected case that they are equal
-				if existing.Forward.Equal(snoopedNames) && existing.Reverse.Equal(reverseNames) {
+				var sUnchanged, rUnchanged bool
+				snoopedNames, sUnchanged = snoopedNames.Merge(existing.Forward)
+				reverseNames, rUnchanged = reverseNames.Merge(existing.Reverse)
+				if sUnchanged && rUnchanged {
 					continue
 				}
-				// Not equal - merge this node's data into existing data,
-				snoopedNames = snoopedNames.Merge(existing.Forward)
-				reverseNames = reverseNames.Merge(existing.Reverse)
 			}
 			dns[addr] = DNSRecord{Forward: snoopedNames, Reverse: reverseNames}
 		}
diff --git a/report/sets.go b/report/sets.go
index 2775dbeaa0..a057275860 100644
--- a/report/sets.go
+++ b/report/sets.go
@@ -35,7 +35,11 @@ func (s Sets) Add(key string, value StringSet) Sets {
 		s = emptySets
 	}
 	if existingValue, ok := s.psMap.Lookup(key); ok {
-		value = value.Merge(existingValue.(StringSet))
+		var unchanged bool
+		value, unchanged = existingValue.(StringSet).Merge(value)
+		if unchanged {
+			return s
+		}
 	}
 	return Sets{
 		psMap: s.psMap.Set(key, value),
@@ -94,7 +98,11 @@ func (s Sets) Merge(other Sets) Sets {
 	iter.ForEach(func(key string, value interface{}) {
 		set := value.(StringSet)
 		if existingSet, ok := result.Lookup(key); ok {
-			set = set.Merge(existingSet.(StringSet))
+			var unchanged bool
+			set, unchanged = existingSet.(StringSet).Merge(set)
+			if unchanged {
+				return
+			}
 		}
 		result = result.Set(key, set)
 	})
diff --git a/report/sets_test.go b/report/sets_test.go
index c2ac164fb1..c29e42e659 100644
--- a/report/sets_test.go
+++ b/report/sets_test.go
@@ -1,12 +1,44 @@
 package report_test
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/weaveworks/scope/report"
 	"github.com/weaveworks/scope/test/reflect"
 )
 
+func TestSetsAdd(t *testing.T) {
+	for _, testcase := range []struct {
+		a    report.Sets
+		want map[string][]string
+	}{
+		{
+			report.MakeSets().Add("a", report.MakeStringSet("b")),
+			map[string][]string{"a": {"b"}},
+		},
+		{
+			report.MakeSets().Add("a", report.MakeStringSet("b")).Add("a", report.MakeStringSet("c")),
+			map[string][]string{"a": {"b", "c"}},
+		},
+		{
+			report.MakeSets().Add("a", report.MakeStringSet("b", "c")).Add("a", report.MakeStringSet("c")),
+			map[string][]string{"a": {"b", "c"}},
+		},
+		{
+			report.MakeSets().Add("a", report.MakeStringSet("c")).Add("a", report.MakeStringSet("b", "c")),
+			map[string][]string{"a": {"b", "c"}},
+		},
+		{
+			report.MakeSets().Add("a", report.MakeStringSet("1")).Add("b", report.MakeStringSet("2")).
+				Add("c", report.MakeStringSet("3")).Add("b", report.MakeStringSet("3")),
+			map[string][]string{"a": {"1"}, "b": {"2", "3"}, "c": {"3"}},
+		},
+	} {
+		check(t, "Add", testcase.a, testcase.want)
+	}
+}
+
 func TestSetsMerge(t *testing.T) {
 	for _, testcase := range []struct {
 		a, b report.Sets
@@ -29,15 +61,19 @@ func TestSetsMerge(t *testing.T) {
 			map[string][]string{"a": {"1"}, "b": {"2", "3"}, "c": {"3"}},
 		},
 	} {
-		haveSets := testcase.a.Merge(testcase.b)
-		have := map[string][]string{}
-		keys := haveSets.Keys()
-		for _, k := range keys {
-			have[k], _ = haveSets.Lookup(k)
-		}
+		check(t, fmt.Sprintf("%+v.Merge(%+v)", testcase.a, testcase.b), testcase.a.Merge(testcase.b), testcase.want)
+		check(t, fmt.Sprintf("%+v.Merge(%+v)", testcase.b, testcase.a), testcase.b.Merge(testcase.a), testcase.want)
+	}
+}
+
+func check(t *testing.T, desc string, haveSets report.Sets, want map[string][]string) {
+	have := map[string][]string{}
+	keys := haveSets.Keys()
+	for _, k := range keys {
+		have[k], _ = haveSets.Lookup(k)
+	}
 
-		if !reflect.DeepEqual(testcase.want, have) {
-			t.Errorf("%+v.Merge(%+v): want %+v, have %+v", testcase.a, testcase.b, testcase.want, have)
-		}
+	if !reflect.DeepEqual(want, have) {
+		t.Errorf("%s: want %+v, have %+v", desc, want, have)
 	}
 }
diff --git a/report/string_set.go b/report/string_set.go
index c20014e314..7580d140ca 100644
--- a/report/string_set.go
+++ b/report/string_set.go
@@ -81,32 +81,54 @@ func (s StringSet) Add(strs ...string) StringSet {
 }
 
 // Merge combines the two StringSets and returns a new result.
-func (s StringSet) Merge(other StringSet) StringSet {
+// Second return value is true if the return value is s
+func (s StringSet) Merge(other StringSet) (StringSet, bool) {
 	switch {
 	case len(other) <= 0: // Optimise special case, to avoid allocating
-		return s // (note unit test DeepEquals breaks if we don't do this)
+		return s, true // (note unit test DeepEquals breaks if we don't do this)
 	case len(s) <= 0:
-		return other
+		return other, false
 	}
-	result := make(StringSet, len(s)+len(other))
-	for i, j, k := 0, 0, 0; ; k++ {
+
+	i, j := 0, 0
+loop:
+	for i < len(s) {
 		switch {
-		case i >= len(s):
-			copy(result[k:], other[j:])
-			return result[:k+len(other)-j]
 		case j >= len(other):
-			copy(result[k:], s[i:])
-			return result[:k+len(s)-i]
+			return s, true
+		case s[i] == other[j]:
+			i++
+			j++
 		case s[i] < other[j]:
-			result[k] = s[i]
 			i++
-		case s[i] > other[j]:
-			result[k] = other[j]
+		default:
+			break loop
+		}
+	}
+	if i >= len(s) && j >= len(other) {
+		return s, true
+	}
+
+	result := make(StringSet, i, len(s)+len(other))
+	copy(result, s[:i])
+
+	for i < len(s) {
+		switch {
+		case j >= len(other):
+			result = append(result, s[i:]...)
+			return result, false
+		case s[i] == other[j]:
+			result = append(result, s[i])
+			i++
 			j++
-		default: // equal
-			result[k] = s[i]
+		case s[i] < other[j]:
+			result = append(result, s[i])
 			i++
+		default:
+			result = append(result, other[j])
 			j++
 		}
 	}
+	result = append(result, other[j:]...)
+	return result, false
 }
diff --git a/report/topology_test.go b/report/topology_test.go
index 08ca1a2b15..f50b2b4608 100644
--- a/report/topology_test.go
+++ b/report/topology_test.go
@@ -61,8 +61,8 @@ func TestStringSetMerge(t *testing.T) {
 		{input: report.MakeStringSet("a", "c"), other: report.MakeStringSet("a", "b"), want: report.MakeStringSet("a", "b", "c")},
 		{input: report.MakeStringSet("b"), other: report.MakeStringSet("a"), want: report.MakeStringSet("a", "b")},
 	} {
-		if want, have := testcase.want, testcase.input.Merge(testcase.other); !reflect.DeepEqual(want, have) {
-			t.Errorf("%v + %v: want %v, have %v", testcase.input, testcase.other, want, have)
+		if have, _ := testcase.input.Merge(testcase.other); !reflect.DeepEqual(testcase.want, have) {
+			t.Errorf("%v + %v: want %v, have %v", testcase.input, testcase.other, testcase.want, have)
 		}
 	}
 }