diff --git a/inmem.go b/inmem.go
index 024d0d1..d1eb9c9 100644
--- a/inmem.go
+++ b/inmem.go
@@ -10,8 +10,6 @@ import (
 	"time"
 )
 
-var spaceReplacer = strings.NewReplacer(" ", "_")
-
 // InmemSink provides a MetricSink that does in-memory aggregation
 // without sending metrics over a network. It can be embedded within
 // an application to provide profiling information.
@@ -338,13 +336,7 @@ func (i *InmemSink) getInterval() *IntervalMetrics {
 
 // Flattens the key for formatting, removes spaces
 func (i *InmemSink) flattenKey(parts []string) string {
-	buf := &bytes.Buffer{}
-
-	joined := strings.Join(parts, ".")
-
-	spaceReplacer.WriteString(buf, joined)
-
-	return buf.String()
+	return strings.Replace(strings.Join(parts, "."), " ", "_", -1)
 }
 
 // Flattens the key for formatting along with its labels, removes spaces
@@ -353,8 +345,11 @@ func (i *InmemSink) flattenKeyLabels(parts []string, labels []Label) (string, st
 	buf := bytes.NewBufferString(key)
 
 	for _, label := range labels {
-		spaceReplacer.WriteString(buf, fmt.Sprintf(";%s=%s", label.Name, label.Value))
+		buf.WriteByte(';')
+		buf.WriteString(label.Name)
+		buf.WriteByte('=')
+		buf.WriteString(label.Value)
 	}
 
-	return buf.String(), key
+	return strings.Replace(buf.String(), " ", "_", -1), key
 }
diff --git a/inmem_test.go b/inmem_test.go
index 3b037c7..0e264aa 100644
--- a/inmem_test.go
+++ b/inmem_test.go
@@ -188,3 +188,69 @@ func duration(t *testing.T, s string) time.Duration {
 	}
 	return dur
 }
+
+func BenchmarkFlattenKey(b *testing.B) {
+	cases := []struct {
+		name  string
+		parts []string
+	}{
+		{
+			name:  "three-segments",
+			parts: []string{"foo", "bar", "baz"},
+		},
+		{
+			name:  "five-segments",
+			parts: []string{"foo", "bar", "baz", "qux", "quux"},
+		},
+		{
+			name:  "ten-segments",
+			parts: []string{"foo", "bar", "baz", "qux", "quux", "corge", "grault", "garply", "waldo", "fred"},
+		},
+	}
+
+	for _, c := range cases {
+		b.Run(c.name, func(b *testing.B) {
+			s := &InmemSink{}
+			for i := 0; i < b.N; i++ {
+				s.flattenKey(c.parts)
+			}
+		})
+	}
+}
+
+func BenchmarkFlattenKeyLabels(b *testing.B) {
+	cases := []struct {
+		name   string
+		parts  []string
+		labels []Label
+	}{
+		{
+			name:  "three-segments-no-labels",
+			parts: []string{"foo", "bar", "baz"},
+		},
+		{
+			name:   "three-segments-one-label",
+			parts:  []string{"foo", "bar", "baz"},
+			labels: []Label{{"blah", "arg"}},
+		},
+		{
+			name:   "five-segments-three-labels",
+			parts:  []string{"foo", "bar", "baz", "qux", "quux"},
+			labels: []Label{{"blah1", "arg1"}, {"blah2", "arg2"}, {"blah3", "arg3"}},
+		},
+		{
+			name:   "ten-segments-five-labels",
+			parts:  []string{"foo", "bar", "baz", "qux", "quux", "corge", "grault", "garply", "waldo", "fred"},
+			labels: []Label{{"blah1", "arg1"}, {"blah2", "arg2"}, {"blah3", "arg3"}, {"blah4", "arg4"}, {"blah5", "arg5"}},
+		},
+	}
+
+	for _, c := range cases {
+		b.Run(c.name, func(b *testing.B) {
+			s := &InmemSink{}
+			for i := 0; i < b.N; i++ {
+				s.flattenKeyLabels(c.parts, c.labels)
+			}
+		})
+	}
+}