diff --git a/pkg/store/prometheus.go b/pkg/store/prometheus.go
index 6f5bd1b7de..f9e836aca9 100644
--- a/pkg/store/prometheus.go
+++ b/pkg/store/prometheus.go
@@ -116,12 +116,11 @@ func (p *PrometheusStore) Info(_ context.Context, _ *storepb.InfoRequest) (*stor
 	mint, maxt := p.timestamps()
 
 	res := &storepb.InfoResponse{
-		Labels:    make([]labelpb.ZLabel, 0, len(lset)),
+		Labels:    labelpb.ZLabelsFromPromLabels(lset),
 		StoreType: p.component.ToProto(),
 		MinTime:   mint,
 		MaxTime:   maxt,
 	}
-	res.Labels = append(res.Labels, labelpb.ZLabelsFromPromLabels(lset)...)
 
 	// Until we deprecate the single labels in the reply, we just duplicate
 	// them here for migration/compatibility purposes.
@@ -191,7 +190,7 @@ func (p *PrometheusStore) Series(r *storepb.SeriesRequest, seriesSrv storepb.Sto
 			return err
 		}
 		for _, lbm := range labelMaps {
-			lset := make([]labelpb.ZLabel, 0, len(lbm)+len(finalExtLset))
+			lset := make([]labelpb.ZLabel, 0, len(lbm)+finalExtLset.Len())
 			for k, v := range lbm {
 				lset = append(lset, labelpb.ZLabel{Name: k, Value: v})
 			}
@@ -290,19 +289,18 @@ func (p *PrometheusStore) queryPrometheus(
 	}
 
 	externalLbls := rmLabels(p.externalLabelsFn().Copy(), extLsetToRemove)
+	b := labels.NewScratchBuilder(16)
 	for _, vector := range matrix {
-		seriesLbls := labels.Labels(make([]labels.Label, 0, len(vector.Metric)))
+		b.Reset()
 
 		// Attach labels from samples.
 		for k, v := range vector.Metric {
-			seriesLbls = append(seriesLbls, labels.FromStrings(string(k), string(v))...)
+			b.Add(string(k), string(v))
 		}
-		sort.Slice(seriesLbls, func(i, j int) bool {
-			return seriesLbls.Less(i, j)
-		})
-		// Attach external labels for compatibility with remote read.
-		finalLbls := labelpb.ExtendSortedLabels(seriesLbls, externalLbls)
-		finalLbls = append(finalLbls, dedup.PushdownMarker)
+		b.Add(dedup.PushdownMarker.Name, dedup.PushdownMarker.Value)
+		b.Sort()
+
+		finalLbls := labelpb.ExtendSortedLabels(b.Labels(), externalLbls)
 
 		series := &prompb.TimeSeries{
 			Labels:  labelpb.ZLabelsFromPromLabels(finalLbls),
@@ -604,7 +602,7 @@ func matchesExternalLabels(ms []storepb.LabelMatcher, externalLabels labels.Labe
 		return false, nil, err
 	}
 
-	if len(externalLabels) == 0 {
+	if externalLabels.IsEmpty() {
 		return true, tms, nil
 	}
 
@@ -682,9 +680,9 @@ func (p *PrometheusStore) LabelNames(ctx context.Context, r *storepb.LabelNamesR
 	}
 
 	if len(lbls) > 0 {
-		for _, extLbl := range extLset {
-			lbls = append(lbls, extLbl.Name)
-		}
+		extLset.Range(func(l labels.Label) {
+			lbls = append(lbls, l.Name)
+		})
 		sort.Strings(lbls)
 	}
 
@@ -746,10 +744,7 @@ func (p *PrometheusStore) LabelValues(ctx context.Context, r *storepb.LabelValue
 }
 
 func (p *PrometheusStore) LabelSet() []labelpb.ZLabelSet {
-	lset := p.externalLabelsFn()
-
-	labels := make([]labelpb.ZLabel, 0, len(lset))
-	labels = append(labels, labelpb.ZLabelsFromPromLabels(lset)...)
+	labels := labelpb.ZLabelsFromPromLabels(p.externalLabelsFn())
 
 	labelset := []labelpb.ZLabelSet{}
 	if len(labels) > 0 {
diff --git a/pkg/store/prometheus_test.go b/pkg/store/prometheus_test.go
index d0597b6e9c..8ed13a9f99 100644
--- a/pkg/store/prometheus_test.go
+++ b/pkg/store/prometheus_test.go
@@ -169,9 +169,9 @@ func testPrometheusStoreSeriesE2e(t *testing.T, prefix string) {
 		testutil.Equals(t, 1, len(srv.SeriesSet))
 
 		testutil.Equals(t, []labelpb.ZLabel{
+			{Name: "__thanos_pushed_down", Value: "true"},
 			{Name: "a", Value: "b"},
 			{Name: "region", Value: "eu-west"},
-			{Name: "__thanos_pushed_down", Value: "true"},
 		}, srv.SeriesSet[0].Labels)
 		testutil.Equals(t, []string(nil), srv.Warnings)
 		testutil.Equals(t, 1, len(srv.SeriesSet[0].Chunks))
diff --git a/pkg/store/storepb/testutil/series.go b/pkg/store/storepb/testutil/series.go
index ffc69d1cc6..a2d43b6c56 100644
--- a/pkg/store/storepb/testutil/series.go
+++ b/pkg/store/storepb/testutil/series.go
@@ -138,7 +138,6 @@ func ReadSeriesFromBlock(t testing.TB, h tsdb.BlockReader, extLabels labels.Labe
 	defer func() { testutil.Ok(t, ir.Close()) }()
 
 	var (
-		lset       labels.Labels
 		chunkMetas []chunks.Meta
 		expected   = make([]*storepb.Series, 0)
 	)
@@ -148,8 +147,8 @@ func ReadSeriesFromBlock(t testing.TB, h tsdb.BlockReader, extLabels labels.Labe
 	all := allPostings(context.TODO(), t, ir)
 	for all.Next() {
 		testutil.Ok(t, ir.Series(all.At(), &builder, &chunkMetas))
-		lset = builder.Labels()
-		expected = append(expected, &storepb.Series{Labels: labelpb.ZLabelsFromPromLabels(append(extLabels.Copy(), lset...))})
+		lset := labelpb.ExtendSortedLabels(builder.Labels(), extLabels)
+		expected = append(expected, &storepb.Series{Labels: labelpb.ZLabelsFromPromLabels(lset)})
 
 		if skipChunks {
 			continue
@@ -189,7 +188,7 @@ func appendFloatSamples(t testing.TB, app storage.Appender, tsLabel int, opts He
 	testutil.Ok(t, err)
 
 	for is := 1; is < opts.SamplesPerSeries; is++ {
-		_, err := app.Append(ref, nil, int64(tsLabel+is)*opts.ScrapeInterval.Milliseconds(), opts.Random.Float64())
+		_, err := app.Append(ref, labels.EmptyLabels(), int64(tsLabel+is)*opts.ScrapeInterval.Milliseconds(), opts.Random.Float64())
 		testutil.Ok(t, err)
 	}
 }
@@ -218,7 +217,7 @@ func appendHistogramSamples(t testing.TB, app storage.Appender, tsLabel int, opt
 	testutil.Ok(t, err)
 
 	for is := 1; is < opts.SamplesPerSeries; is++ {
-		_, err := app.AppendHistogram(ref, nil, int64(tsLabel+is)*opts.ScrapeInterval.Milliseconds(), sample, nil)
+		_, err := app.AppendHistogram(ref, labels.EmptyLabels(), int64(tsLabel+is)*opts.ScrapeInterval.Milliseconds(), sample, nil)
 		testutil.Ok(t, err)
 	}
 }
diff --git a/pkg/testutil/e2eutil/prometheus.go b/pkg/testutil/e2eutil/prometheus.go
index b0f1fc6576..6ae618a199 100644
--- a/pkg/testutil/e2eutil/prometheus.go
+++ b/pkg/testutil/e2eutil/prometheus.go
@@ -509,10 +509,6 @@ func createBlock(
 				app := h.Appender(ctx)
 
 				for _, lset := range batch {
-					sort.Slice(lset, func(i, j int) bool {
-						return lset[i].Name < lset[j].Name
-					})
-
 					var err error
 					if sampleType == chunkenc.ValFloat {
 						randMutex.Lock()