diff --git a/storage/buffer_test.go b/storage/buffer_test.go
index a03081ab6a..2a1b872489 100644
--- a/storage/buffer_test.go
+++ b/storage/buffer_test.go
@@ -201,6 +201,17 @@ type mockSeries struct {
 	iterator func() SeriesIterator
 }
 
+func newMockSeries(lset labels.Labels, samples []sample) Series {
+	return &mockSeries{
+		labels: func() labels.Labels {
+			return lset
+		},
+		iterator: func() SeriesIterator {
+			return newListSeriesIterator(samples)
+		},
+	}
+}
+
 func (m *mockSeries) Labels() labels.Labels    { return m.labels() }
 func (m *mockSeries) Iterator() SeriesIterator { return m.iterator() }
 
diff --git a/storage/fanout.go b/storage/fanout.go
index ae74ad2ca2..40be2536d6 100644
--- a/storage/fanout.go
+++ b/storage/fanout.go
@@ -303,6 +303,10 @@ type mergeSeriesSet struct {
 // NewMergeSeriesSet returns a new series set that merges (deduplicates)
 // series returned by the input series sets when iterating.
 func NewMergeSeriesSet(sets []SeriesSet) SeriesSet {
+	if len(sets) == 1 {
+		return sets[0]
+	}
+
 	// Sets need to be pre-advanced, so we can introspect the label of the
 	// series under the cursor.
 	var h seriesSetHeap
@@ -340,6 +344,9 @@ func (c *mergeSeriesSet) Next() bool {
 }
 
 func (c *mergeSeriesSet) At() Series {
+	if len(c.currentSets) == 1 {
+		return c.currentSets[0].At()
+	}
 	series := []Series{}
 	for _, seriesSet := range c.currentSets {
 		series = append(series, seriesSet.At())
diff --git a/storage/fanout_test.go b/storage/fanout_test.go
index c8094ea333..66d86fc938 100644
--- a/storage/fanout_test.go
+++ b/storage/fanout_test.go
@@ -14,6 +14,7 @@
 package storage
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -218,13 +219,53 @@ func (m *mockSeriesSet) Err() error {
 	return nil
 }
 
-func newMockSeries(lset labels.Labels, samples []sample) Series {
-	return &mockSeries{
-		labels: func() labels.Labels {
-			return lset
-		},
-		iterator: func() SeriesIterator {
-			return newListSeriesIterator(samples)
-		},
+var result []sample
+
+func makeSeriesSet(numSeries, numSamples int) SeriesSet {
+	series := []Series{}
+	for j := 0; j < numSeries; j++ {
+		labels := labels.Labels{{Name: "foo", Value: fmt.Sprintf("bar%d", j)}}
+		samples := []sample{}
+		for k := 0; k < numSamples; k++ {
+			samples = append(samples, sample{t: int64(k), v: float64(k)})
+		}
+		series = append(series, newMockSeries(labels, samples))
+	}
+	return newMockSeriesSet(series...)
+}
+
+func makeMergeSeriesSet(numSeriesSets, numSeries, numSamples int) SeriesSet {
+	seriesSets := []SeriesSet{}
+	for i := 0; i < numSeriesSets; i++ {
+		seriesSets = append(seriesSets, makeSeriesSet(numSeries, numSamples))
+	}
+	return NewMergeSeriesSet(seriesSets)
+}
+
+func benchmarkDrain(seriesSet SeriesSet, b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		for seriesSet.Next() {
+			result = drainSamples(seriesSet.At().Iterator())
+		}
+	}
+}
+
+func BenchmarkNoMergeSeriesSet_100_100(b *testing.B) {
+	seriesSet := makeSeriesSet(100, 100)
+	benchmarkDrain(seriesSet, b)
+}
+
+func BenchmarkMergeSeriesSet(b *testing.B) {
+	for _, bm := range []struct {
+		numSeriesSets, numSeries, numSamples int
+	}{
+		{1, 100, 100},
+		{10, 100, 100},
+		{100, 100, 100},
+	} {
+		seriesSet := makeMergeSeriesSet(bm.numSeriesSets, bm.numSeries, bm.numSamples)
+		b.Run(fmt.Sprintf("%d_%d_%d", bm.numSeriesSets, bm.numSeries, bm.numSamples), func(b *testing.B) {
+			benchmarkDrain(seriesSet, b)
+		})
 	}
 }