diff --git a/head.go b/head.go
index 211cadf77..d9b4589ee 100644
--- a/head.go
+++ b/head.go
@@ -59,6 +59,10 @@ type headBlock struct {
 
 	metamtx sync.RWMutex
 	meta    BlockMeta
+
+	updatec chan struct{}
+	stopc   chan struct{}
+	donec   chan struct{}
 }
 
 func createHeadBlock(dir string, seq int, l log.Logger, mint, maxt int64) (*headBlock, error) {
@@ -107,6 +111,9 @@ func openHeadBlock(dir string, l log.Logger) (*headBlock, error) {
 		postings: &memPostings{m: make(map[term][]uint32)},
 		mapper:   newPositionMapper(nil),
 		meta:     *meta,
+		updatec:  make(chan struct{}, 1),
+		stopc:    make(chan struct{}),
+		donec:    make(chan struct{}),
 	}
 
 	r := wal.Reader()
@@ -131,11 +138,25 @@ func openHeadBlock(dir string, l log.Logger) (*headBlock, error) {
 		return nil, errors.Wrap(err, "consume WAL")
 	}
 
-	h.updateMapping()
+	go h.run()
+	h.updatec <- struct{}{}
 
 	return h, nil
 }
 
+func (h *headBlock) run() {
+	defer close(h.donec)
+
+	for {
+		select {
+		case <-h.stopc:
+			return
+		case <-h.updatec:
+			h.updateMapping()
+		}
+	}
+}
+
 // inBounds returns true if the given timestamp is within the valid
 // time bounds of the block.
 func (h *headBlock) inBounds(t int64) bool {
@@ -144,6 +165,9 @@ func (h *headBlock) inBounds(t int64) bool {
 
 // Close syncs all data and closes underlying resources of the head block.
 func (h *headBlock) Close() error {
+	close(h.stopc)
+	<-h.donec
+
 	// Lock mutex and leave it locked so we panic if there's a bug causing
 	// the block to be used afterwards.
 	h.mtx.Lock()
@@ -324,6 +348,11 @@ func (a *headAppender) createSeries() {
 
 	a.mtx.Unlock()
 	a.mtx.RLock()
+
+	select {
+	case a.updatec <- struct{}{}:
+	default:
+	}
 }
 
 func (a *headAppender) Commit() error {
@@ -530,6 +559,8 @@ func (h *headBlock) create(hash uint64, lset labels.Labels) *memSeries {
 func (h *headBlock) updateMapping() {
 	h.mtx.RLock()
 
+	// No need to rlock the mapper as this method is not run concurrently and
+	// the only one ever modifying the mapper.
 	if h.mapper.sortable != nil && h.mapper.Len() == len(h.series) {
 		h.mtx.RUnlock()
 		return
@@ -544,7 +575,9 @@ func (h *headBlock) updateMapping() {
 		return labels.Compare(series[i].lset, series[j].lset) < 0
 	})
 
+	h.mapper.mtx.Lock()
 	h.mapper.update(s)
+	h.mapper.mtx.Unlock()
 }
 
 // remapPostings changes the order of the postings from their ID to the ordering
@@ -552,18 +585,28 @@ func (h *headBlock) updateMapping() {
 // Returned postings have no longer monotonic IDs and MUST NOT be used for regular
 // postings set operations, i.e. intersect and merge.
 func (h *headBlock) remapPostings(p Postings) Postings {
-	list, err := expandPostings(p)
-	if err != nil {
-		return errPostings{err: err}
-	}
+	// Expand the postings but only up until the point where the mapper
+	// covers existing metrics.
+	ep := make([]uint32, 0, 64)
 
-	h.mapper.mtx.Lock()
-	defer h.mapper.mtx.Unlock()
+	h.mapper.mtx.RLock()
+	defer h.mapper.mtx.RUnlock()
+
+	max := uint32(h.mapper.Len())
+
+	for p.Next() {
+		if p.At() > max {
+			break
+		}
+		ep = append(ep, p.At())
+	}
+	if err := p.Err(); err != nil {
+		return errPostings{err: errors.Wrap(err, "expand postings")}
+	}
 
-	h.updateMapping()
-	h.mapper.Sort(list)
+	h.mapper.Sort(ep)
 
-	return newListPostings(list)
+	return newListPostings(ep)
 }
 
 type memSeries struct {