diff --git a/tsdb/head.go b/tsdb/head.go
index 575177a7a..410a226d8 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -1667,26 +1667,34 @@ func (h *Head) mmapHeadChunks() {
 	var count int
 	for i := 0; i < h.series.size; i++ {
 		h.series.locks[i].RLock()
-		for _, all := range h.series.hashes[i] {
-			for _, series := range all {
-				series.Lock()
-				count += series.mmapChunks(h.chunkDiskMapper)
-				series.Unlock()
-			}
+		for _, series := range h.series.series[i] {
+			series.Lock()
+			count += series.mmapChunks(h.chunkDiskMapper)
+			series.Unlock()
 		}
 		h.series.locks[i].RUnlock()
 	}
 	h.metrics.mmapChunksTotal.Add(float64(count))
 }
 
-// seriesHashmap is a simple hashmap for memSeries by their label set. It is built
-// on top of a regular hashmap and holds a slice of series to resolve hash collisions.
+// seriesHashmap lets TSDB find a memSeries by its label set, via a 64-bit hash.
+// There is one map for the common case where the hash value is unique, and a
+// second map for the case that two series have the same hash value.
+// Each series is in only one of the maps.
 // Its methods require the hash to be submitted with it to avoid re-computations throughout
 // the code.
-type seriesHashmap map[uint64][]*memSeries
+type seriesHashmap struct {
+	unique    map[uint64]*memSeries
+	conflicts map[uint64][]*memSeries
+}
 
-func (m seriesHashmap) get(hash uint64, lset labels.Labels) *memSeries {
-	for _, s := range m[hash] {
+func (m *seriesHashmap) get(hash uint64, lset labels.Labels) *memSeries {
+	if s, found := m.unique[hash]; found {
+		if labels.Equal(s.lset, lset) {
+			return s
+		}
+	}
+	for _, s := range m.conflicts[hash] {
 		if labels.Equal(s.lset, lset) {
 			return s
 		}
@@ -1695,27 +1703,46 @@ func (m seriesHashmap) get(hash uint64, lset labels.Labels) *memSeries {
 }
 
 func (m seriesHashmap) set(hash uint64, s *memSeries) {
-	l := m[hash]
+	if existing, found := m.unique[hash]; !found || labels.Equal(existing.lset, s.lset) {
+		m.unique[hash] = s
+		return
+	}
+	l := m.conflicts[hash]
 	for i, prev := range l {
 		if labels.Equal(prev.lset, s.lset) {
 			l[i] = s
 			return
 		}
 	}
-	m[hash] = append(l, s)
+	m.conflicts[hash] = append(l, s)
 }
 
 func (m seriesHashmap) del(hash uint64, lset labels.Labels) {
 	var rem []*memSeries
-	for _, s := range m[hash] {
-		if !labels.Equal(s.lset, lset) {
-			rem = append(rem, s)
+	unique, found := m.unique[hash]
+	switch {
+	case !found:
+		return
+	case labels.Equal(unique.lset, lset):
+		conflicts := m.conflicts[hash]
+		if len(conflicts) == 0 {
+			delete(m.unique, hash)
+			return
+		}
+		rem = conflicts
+	default:
+		rem = append(rem, unique)
+		for _, s := range m.conflicts[hash] {
+			if !labels.Equal(s.lset, lset) {
+				rem = append(rem, s)
+			}
 		}
 	}
-	if len(rem) == 0 {
-		delete(m, hash)
+	m.unique[hash] = rem[0]
+	if len(rem) == 1 {
+		delete(m.conflicts, hash)
 	} else {
-		m[hash] = rem
+		m.conflicts[hash] = rem[1:]
 	}
 }
 
@@ -1757,7 +1784,10 @@ func newStripeSeries(stripeSize int, seriesCallback SeriesLifecycleCallback) *st
 		s.series[i] = map[chunks.HeadSeriesRef]*memSeries{}
 	}
 	for i := range s.hashes {
-		s.hashes[i] = seriesHashmap{}
+		s.hashes[i] = seriesHashmap{
+			unique:    map[uint64]*memSeries{},
+			conflicts: map[uint64][]*memSeries{},
+		}
 	}
 	return s
 }
@@ -1837,7 +1867,10 @@ func (s *stripeSeries) gc(mint int64, minOOOMmapRef chunks.ChunkDiskMapperRef) (
 		deletedForCallback := make(map[chunks.HeadSeriesRef]labels.Labels, deletedFromPrevStripe)
 		s.locks[i].Lock()
 
-		for hash, all := range s.hashes[i] {
+		for hash, series := range s.hashes[i].unique {
+			check(i, hash, series, deletedForCallback)
+		}
+		for hash, all := range s.hashes[i].conflicts {
 			for _, series := range all {
 				check(i, hash, series, deletedForCallback)
 			}