diff --git a/internal/repository/repository.go b/internal/repository/repository.go
index 0d5242022..f5d07a08b 100644
--- a/internal/repository/repository.go
+++ b/internal/repository/repository.go
@@ -111,29 +111,33 @@ func (r *Repository) LoadAndDecrypt(ctx context.Context, buf []byte, t restic.Fi
 	return plaintext, nil
 }
 
-// sortCachedPacks moves all cached pack files to the front of blobs.
-func (r *Repository) sortCachedPacks(blobs []restic.PackedBlob) []restic.PackedBlob {
-	if r.Cache == nil {
-		return blobs
+type haver interface {
+	Has(restic.Handle) bool
+}
+
+// sortCachedPacksFirst moves all cached pack files to the front of blobs.
+func sortCachedPacksFirst(cache haver, blobs []restic.PackedBlob) {
+	if cache == nil {
+		return
 	}
 
 	// no need to sort a list with one element
 	if len(blobs) == 1 {
-		return blobs
+		return
 	}
 
-	cached := make([]restic.PackedBlob, 0, len(blobs)/2)
+	cached := blobs[:0]
 	noncached := make([]restic.PackedBlob, 0, len(blobs)/2)
 
 	for _, blob := range blobs {
-		if r.Cache.Has(restic.Handle{Type: restic.DataFile, Name: blob.PackID.String()}) {
+		if cache.Has(restic.Handle{Type: restic.DataFile, Name: blob.PackID.String()}) {
 			cached = append(cached, blob)
 			continue
 		}
 		noncached = append(noncached, blob)
 	}
 
-	return append(cached, noncached...)
+	copy(blobs[len(cached):], noncached)
 }
 
 // LoadBlob loads a blob of type t from the repository.
@@ -149,7 +153,7 @@ func (r *Repository) LoadBlob(ctx context.Context, t restic.BlobType, id restic.
 	}
 
 	// try cached pack files first
-	blobs = r.sortCachedPacks(blobs)
+	sortCachedPacksFirst(r.Cache, blobs)
 
 	var lastError error
 	for _, blob := range blobs {
diff --git a/internal/repository/repository_internal_test.go b/internal/repository/repository_internal_test.go
new file mode 100644
index 000000000..7bd126b77
--- /dev/null
+++ b/internal/repository/repository_internal_test.go
@@ -0,0 +1,74 @@
+package repository
+
+import (
+	"math/rand"
+	"sort"
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+type mapcache map[restic.Handle]bool
+
+func (c mapcache) Has(h restic.Handle) bool { return c[h] }
+
+func TestSortCachedPacksFirst(t *testing.T) {
+	var (
+		blobs, sorted [100]restic.PackedBlob
+
+		cache = make(mapcache)
+		r     = rand.New(rand.NewSource(1261))
+	)
+
+	for i := 0; i < len(blobs); i++ {
+		var id restic.ID
+		r.Read(id[:])
+		blobs[i] = restic.PackedBlob{PackID: id}
+
+		if i%3 == 0 {
+			h := restic.Handle{Name: id.String(), Type: restic.DataFile}
+			cache[h] = true
+		}
+	}
+
+	copy(sorted[:], blobs[:])
+	sort.SliceStable(sorted[:], func(i, j int) bool {
+		hi := restic.Handle{Type: restic.DataFile, Name: sorted[i].PackID.String()}
+		hj := restic.Handle{Type: restic.DataFile, Name: sorted[j].PackID.String()}
+		return cache.Has(hi) && !cache.Has(hj)
+	})
+
+	sortCachedPacksFirst(cache, blobs[:])
+	rtest.Equals(t, sorted, blobs)
+}
+
+func BenchmarkSortCachedPacksFirst(b *testing.B) {
+	const nblobs = 512 // Corresponds to a file of ca. 2GB.
+
+	var (
+		blobs [nblobs]restic.PackedBlob
+		cache = make(mapcache)
+		r     = rand.New(rand.NewSource(1261))
+	)
+
+	for i := 0; i < nblobs; i++ {
+		var id restic.ID
+		r.Read(id[:])
+		blobs[i] = restic.PackedBlob{PackID: id}
+
+		if i%3 == 0 {
+			h := restic.Handle{Name: id.String(), Type: restic.DataFile}
+			cache[h] = true
+		}
+	}
+
+	var cpy [nblobs]restic.PackedBlob
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		copy(cpy[:], blobs[:])
+		sortCachedPacksFirst(cache, cpy[:])
+	}
+}