diff --git a/db.go b/db.go
index 34d1e884b..a376f465e 100644
--- a/db.go
+++ b/db.go
@@ -491,22 +491,13 @@ type dbAppender struct {
 	samples int
 }
 
-func (a *dbAppender) getAppender(ulid string) (*metaAppender, bool) {
-	for _, h := range a.heads {
-		if string(h.meta.ULID[:]) == ulid {
-			return h, true
-		}
-	}
-	return nil, false
-}
-
 type metaAppender struct {
 	meta BlockMeta
 	app  Appender
 }
 
 func (a *dbAppender) Add(lset labels.Labels, t int64, v float64) (string, error) {
-	h, err := a.appenderFor(t)
+	h, err := a.appenderAt(t)
 	if err != nil {
 		return "", err
 	}
@@ -524,9 +515,13 @@ func (a *dbAppender) AddFast(ref string, t int64, v float64) error {
 		return errors.Wrap(ErrNotFound, "invalid ref length")
 	}
 	// The first 16 bytes a ref hold the ULID of the head block.
-	h, ok := a.getAppender(ref[:16])
-	if !ok {
-		return errors.Wrapf(ErrNotFound, "no block for ULID %s", ref[:16])
+	h, err := a.appenderAt(t)
+	if err != nil {
+		return err
+	}
+	// Validate the ref points to the same block we got for t.
+	if string(h.meta.ULID[:]) != ref[:16] {
+		return ErrNotFound
 	}
 	if err := h.app.AddFast(ref[16:], t, v); err != nil {
 		// The block the ref points to might fit the given timestamp.
@@ -543,7 +538,7 @@ func (a *dbAppender) AddFast(ref string, t int64, v float64) error {
 
 // appenderFor gets the appender for the head containing timestamp t.
 // If the head block doesn't exist yet, it gets created.
-func (a *dbAppender) appenderFor(t int64) (*metaAppender, error) {
+func (a *dbAppender) appenderAt(t int64) (*metaAppender, error) {
 	for _, h := range a.heads {
 		if intervalContains(h.meta.MinTime, h.meta.MaxTime-1, t) {
 			return h, nil
@@ -590,13 +585,20 @@ func rangeForTimestamp(t int64, width int64) (mint, maxt int64) {
 // ensureHead makes sure that there is a head block for the timestamp t if
 // it is within or after the currently appendable window.
 func (db *DB) ensureHead(t int64) error {
-	mint, maxt := rangeForTimestamp(t, int64(db.opts.MinBlockDuration))
+	var (
+		mint, maxt = rangeForTimestamp(t, int64(db.opts.MinBlockDuration))
+		addBuffer  = len(db.blocks) == 0
+		last       BlockMeta
+	)
 
-	last := db.blocks[len(db.blocks)-1].Meta()
+	if !addBuffer {
+		last = db.blocks[len(db.blocks)-1].Meta()
+		addBuffer = last.MaxTime <= mint-int64(db.opts.MinBlockDuration)
+	}
 	// Create another block of buffer in front if the DB is initialized or retrieving
 	// new data after a long gap.
 	// This ensures we always have a full block width if append window.
-	if len(db.blocks) == 0 || last.MaxTime <= mint-int64(db.opts.MinBlockDuration) {
+	if addBuffer {
 		if _, err := db.createHeadBlock(mint-int64(db.opts.MinBlockDuration), mint); err != nil {
 			return err
 		}
@@ -621,14 +623,22 @@ func (a *dbAppender) Commit() error {
 
 	// Commits to partial appenders must be concurrent as concurrent appenders
 	// may have conflicting locks on head appenders.
-	// XXX(fabxc): is this a leaky abstraction? Should make an effort to catch a multi-error?
-	var g errgroup.Group
+	// For high-throughput use cases the errgroup causes significant blocking. Typically,
+	// we just deal with a single appender and special case it.
+	var err error
 
-	for _, h := range a.heads {
-		g.Go(h.app.Commit)
+	switch len(a.heads) {
+	case 1:
+		err = a.heads[0].app.Commit()
+	default:
+		var g errgroup.Group
+		for _, h := range a.heads {
+			g.Go(h.app.Commit)
+		}
+		err = g.Wait()
 	}
 
-	if err := g.Wait(); err != nil {
+	if err != nil {
 		return err
 	}
 	// XXX(fabxc): Push the metric down into head block to account properly