diff --git a/head.go b/head.go
index c522907785..0a67400e0c 100644
--- a/head.go
+++ b/head.go
@@ -237,22 +237,28 @@ func NewHead(r prometheus.Registerer, l log.Logger, wal *wal.WAL, chunkRange int
 // Samples before the mint timestamp are discarded.
 func (h *Head) processWALSamples(
 	minValidTime int64,
-	partition, total uint64,
 	input <-chan []RefSample, output chan<- []RefSample,
 ) (unknownRefs uint64) {
 	defer close(output)
 
+	// Mitigate lock contention in getByID.
+	refSeries := map[uint64]*memSeries{}
+
 	mint, maxt := int64(math.MaxInt64), int64(math.MinInt64)
 
 	for samples := range input {
 		for _, s := range samples {
-			if s.T < minValidTime || s.Ref%total != partition {
+			if s.T < minValidTime {
 				continue
 			}
-			ms := h.series.getByID(s.Ref)
+			ms := refSeries[s.Ref]
 			if ms == nil {
-				unknownRefs++
-				continue
+				ms = h.series.getByID(s.Ref)
+				if ms == nil {
+					unknownRefs++
+					continue
+				}
+				refSeries[s.Ref] = ms
 			}
 			_, chunkCreated := ms.append(s.T, s.V)
 			if chunkCreated {
@@ -310,25 +316,22 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 	// They are connected through a ring of channels which ensures that all sample batches
 	// read from the WAL are processed in order.
 	var (
-		wg         sync.WaitGroup
-		n          = runtime.GOMAXPROCS(0)
-		firstInput = make(chan []RefSample, 300)
-		input      = firstInput
+		wg      sync.WaitGroup
+		n       = runtime.GOMAXPROCS(0)
+		inputs  = make([]chan []RefSample, n)
+		outputs = make([]chan []RefSample, n)
 	)
 	wg.Add(n)
 
 	for i := 0; i < n; i++ {
-		output := make(chan []RefSample, 300)
+		outputs[i] = make(chan []RefSample, 300)
+		inputs[i] = make(chan []RefSample, 300)
 
-		go func(i int, input <-chan []RefSample, output chan<- []RefSample) {
-			unknown := h.processWALSamples(minValidTime, uint64(i), uint64(n), input, output)
+		go func(input <-chan []RefSample, output chan<- []RefSample) {
+			unknown := h.processWALSamples(minValidTime, input, output)
 			atomic.AddUint64(&unknownRefs, unknown)
 			wg.Done()
-		}(i, input, output)
-
-		// The output feeds the next worker goroutine. For the last worker,
-		// it feeds the initial input again to reuse the RefSample slices.
-		input = output
+		}(inputs[i], outputs[i])
 	}
 
 	var (
@@ -336,6 +339,7 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 		series  []RefSeries
 		samples []RefSample
 		tstones []Stone
+		err     error
 	)
 	for r.Next() {
 		series, samples, tstones = series[:0], samples[:0], tstones[:0]
@@ -343,7 +347,7 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 
 		switch dec.Type(rec) {
 		case RecordSeries:
-			series, err := dec.Series(rec, series)
+			series, err = dec.Series(rec, series)
 			if err != nil {
 				return errors.Wrap(err, "decode series")
 			}
@@ -355,7 +359,8 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 				}
 			}
 		case RecordSamples:
-			samples, err := dec.Samples(rec, samples)
+			samples, err = dec.Samples(rec, samples)
+			s := samples
 			if err != nil {
 				return errors.Wrap(err, "decode samples")
 			}
@@ -364,20 +369,31 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 			// cause thousands of very large in flight buffers occupying large amounts
 			// of unused memory.
 			for len(samples) > 0 {
-				n := 5000
-				if len(samples) < n {
-					n = len(samples)
+				m := 5000
+				if len(samples) < m {
+					m = len(samples)
 				}
-				var buf []RefSample
-				select {
-				case buf = <-input:
-				default:
+				shards := make([][]RefSample, n)
+				for i := 0; i < n; i++ {
+					var buf []RefSample
+					select {
+					case buf = <-outputs[i]:
+					default:
+					}
+					shards[i] = buf[:0]
 				}
-				firstInput <- append(buf[:0], samples[:n]...)
-				samples = samples[n:]
+				for _, sam := range samples[:m] {
+					mod := sam.Ref % uint64(n)
+					shards[mod] = append(shards[mod], sam)
+				}
+				for i := 0; i < n; i++ {
+					inputs[i] <- shards[i]
+				}
+				samples = samples[m:]
 			}
+			samples = s // Keep whole slice for reuse.
 		case RecordTombstones:
-			tstones, err := dec.Tombstones(rec, tstones)
+			tstones, err = dec.Tombstones(rec, tstones)
 			if err != nil {
 				return errors.Wrap(err, "decode tombstones")
 			}
@@ -397,9 +413,11 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 		return errors.Wrap(r.Err(), "read records")
 	}
 
-	// Signal termination to first worker and wait for last one to close its output channel.
-	close(firstInput)
-	for range input {
+	// Signal termination to each worker and wait for it to close its output channel.
+	for i := 0; i < n; i++ {
+		close(inputs[i])
+		for range outputs[i] {
+		}
 	}
 	wg.Wait()
 
@@ -1341,6 +1359,7 @@ type memSeries struct {
 	ref          uint64
 	lset         labels.Labels
 	chunks       []*memChunk
+	headChunk    *memChunk
 	chunkRange   int64
 	firstChunkID int
 
@@ -1374,6 +1393,7 @@ func (s *memSeries) cut(mint int64) *memChunk {
 		maxTime: math.MinInt64,
 	}
 	s.chunks = append(s.chunks, c)
+	s.headChunk = c
 
 	// Set upper bound on when the next chunk must be started. An earlier timestamp
 	// may be chosen dynamically at a later point.
@@ -1442,6 +1462,11 @@ func (s *memSeries) truncateChunksBefore(mint int64) (removed int) {
 	}
 	s.chunks = append(s.chunks[:0], s.chunks[k:]...)
 	s.firstChunkID += k
+	if len(s.chunks) == 0 {
+		s.headChunk = nil
+	} else {
+		s.headChunk = s.chunks[len(s.chunks)-1]
+	}
 
 	return k
 }
@@ -1524,10 +1549,7 @@ func (s *memSeries) iterator(id int) chunkenc.Iterator {
 }
 
 func (s *memSeries) head() *memChunk {
-	if len(s.chunks) == 0 {
-		return nil
-	}
-	return s.chunks[len(s.chunks)-1]
+	return s.headChunk
 }
 
 type memChunk struct {