diff --git a/tsdb/head.go b/tsdb/head.go
index d3b2b09cce..419340506d 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -1666,26 +1666,34 @@ 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
 		}
@@ -1694,27 +1702,49 @@ 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
+	}
+	if m.conflicts == nil {
+		m.conflicts = make(map[uint64][]*memSeries)
+	}
+	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:]
 	}
 }
 
@@ -1756,7 +1786,10 @@ 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: nil, // Initialized on demand in set().
+		}
 	}
 	return s
 }
@@ -1776,70 +1809,72 @@ func (s *stripeSeries) gc(mint int64, minOOOMmapRef chunks.ChunkDiskMapperRef) (
 		deletedFromPrevStripe       = 0
 	)
 	minMmapFile = math.MaxInt32
-	// Run through all series and truncate old chunks. Mark those with no
-	// chunks left as deleted and store their ID.
+
+	// For one series, truncate old chunks and check if any chunks left. If not, mark as deleted and collect the ID.
+	check := func(hashShard int, hash uint64, series *memSeries, deletedForCallback map[chunks.HeadSeriesRef]labels.Labels) {
+		series.Lock()
+		defer series.Unlock()
+
+		rmChunks += series.truncateChunksBefore(mint, minOOOMmapRef)
+
+		if len(series.mmappedChunks) > 0 {
+			seq, _ := series.mmappedChunks[0].ref.Unpack()
+			if seq < minMmapFile {
+				minMmapFile = seq
+			}
+		}
+		if series.ooo != nil && len(series.ooo.oooMmappedChunks) > 0 {
+			seq, _ := series.ooo.oooMmappedChunks[0].ref.Unpack()
+			if seq < minMmapFile {
+				minMmapFile = seq
+			}
+			for _, ch := range series.ooo.oooMmappedChunks {
+				if ch.minTime < minOOOTime {
+					minOOOTime = ch.minTime
+				}
+			}
+		}
+		if series.ooo != nil && series.ooo.oooHeadChunk != nil {
+			if series.ooo.oooHeadChunk.minTime < minOOOTime {
+				minOOOTime = series.ooo.oooHeadChunk.minTime
+			}
+		}
+		if len(series.mmappedChunks) > 0 || series.headChunks != nil || series.pendingCommit ||
+			(series.ooo != nil && (len(series.ooo.oooMmappedChunks) > 0 || series.ooo.oooHeadChunk != nil)) {
+			seriesMint := series.minTime()
+			if seriesMint < actualMint {
+				actualMint = seriesMint
+			}
+			return
+		}
+		// The series is gone entirely. We need to keep the series lock
+		// and make sure we have acquired the stripe locks for hash and ID of the
+		// series alike.
+		// If we don't hold them all, there's a very small chance that a series receives
+		// samples again while we are half-way into deleting it.
+		refShard := int(series.ref) & (s.size - 1)
+		if hashShard != refShard {
+			s.locks[refShard].Lock()
+			defer s.locks[refShard].Unlock()
+		}
+
+		deleted[storage.SeriesRef(series.ref)] = struct{}{}
+		s.hashes[hashShard].del(hash, series.lset)
+		delete(s.series[refShard], series.ref)
+		deletedForCallback[series.ref] = series.lset
+	}
+
+	// Run through all series shard by shard, checking which should be deleted.
 	for i := 0; i < s.size; i++ {
 		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 {
-				series.Lock()
-				rmChunks += series.truncateChunksBefore(mint, minOOOMmapRef)
-
-				if len(series.mmappedChunks) > 0 {
-					seq, _ := series.mmappedChunks[0].ref.Unpack()
-					if seq < minMmapFile {
-						minMmapFile = seq
-					}
-				}
-				if series.ooo != nil && len(series.ooo.oooMmappedChunks) > 0 {
-					seq, _ := series.ooo.oooMmappedChunks[0].ref.Unpack()
-					if seq < minMmapFile {
-						minMmapFile = seq
-					}
-					for _, ch := range series.ooo.oooMmappedChunks {
-						if ch.minTime < minOOOTime {
-							minOOOTime = ch.minTime
-						}
-					}
-				}
-				if series.ooo != nil && series.ooo.oooHeadChunk != nil {
-					if series.ooo.oooHeadChunk.minTime < minOOOTime {
-						minOOOTime = series.ooo.oooHeadChunk.minTime
-					}
-				}
-				if len(series.mmappedChunks) > 0 || series.headChunks != nil || series.pendingCommit ||
-					(series.ooo != nil && (len(series.ooo.oooMmappedChunks) > 0 || series.ooo.oooHeadChunk != nil)) {
-					seriesMint := series.minTime()
-					if seriesMint < actualMint {
-						actualMint = seriesMint
-					}
-					series.Unlock()
-					continue
-				}
-
-				// The series is gone entirely. We need to keep the series lock
-				// and make sure we have acquired the stripe locks for hash and ID of the
-				// series alike.
-				// If we don't hold them all, there's a very small chance that a series receives
-				// samples again while we are half-way into deleting it.
-				j := int(series.ref) & (s.size - 1)
-
-				if i != j {
-					s.locks[j].Lock()
-				}
-
-				deleted[storage.SeriesRef(series.ref)] = struct{}{}
-				s.hashes[i].del(hash, series.lset)
-				delete(s.series[j], series.ref)
-				deletedForCallback[series.ref] = series.lset
-
-				if i != j {
-					s.locks[j].Unlock()
-				}
-
-				series.Unlock()
+				check(i, hash, series, deletedForCallback)
 			}
 		}