diff --git a/cmd/flatten/main.go b/cmd/flatten/main.go
index e72887a..38052f8 100644
--- a/cmd/flatten/main.go
+++ b/cmd/flatten/main.go
@@ -23,13 +23,13 @@ import (
 	"strings"
 
 	"github.com/google/go-containerregistry/pkg/name"
-	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/empty"
 	"github.com/google/go-containerregistry/pkg/v1/mutate"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
 	"github.com/google/go-containerregistry/pkg/v1/tarball"
 	"github.com/google/go-containerregistry/pkg/v1/types"
 	"github.com/imjasonh/kontain.me/pkg/serve"
+	"golang.org/x/sync/errgroup"
 )
 
 func main() {
@@ -126,31 +126,38 @@ func (s *server) serveFlattenManifest(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		// Flatten each image in the manifest.
-		var fidx v1.ImageIndex = empty.Index
-		for _, m := range im.Manifests {
-			img, err := idx.Image(m.Digest)
-			if err != nil {
-				s.error.Printf("ERROR (idx.Image): %v", err)
-				serve.Error(w, err)
-				return
-			}
-			l, err := tarball.LayerFromOpener(func() (io.ReadCloser, error) { return mutate.Extract(img), nil })
-			if err != nil {
-				s.error.Printf("ERROR (tarball.LayerFromOpener): %v", err)
-				serve.Error(w, err)
-				return
-			}
-			fimg, err := mutate.AppendLayers(empty.Image, l)
-			if err != nil {
-				s.error.Printf("ERROR (mutate.AppendLayers): %v", err)
-				serve.Error(w, err)
-				return
-			}
-			fidx = mutate.AppendManifests(fidx, mutate.IndexAddendum{
-				Add:        fimg,
-				Descriptor: m,
+		var g errgroup.Group
+		adds := make([]mutate.IndexAddendum, len(im.Manifests))
+		for i, m := range im.Manifests {
+			i, m := i, m
+			g.Go(func() error {
+				img, err := idx.Image(m.Digest)
+				if err != nil {
+					s.error.Printf("ERROR (idx.Image): %v", err)
+					return err
+				}
+				l, err := tarball.LayerFromOpener(func() (io.ReadCloser, error) { return mutate.Extract(img), nil })
+				if err != nil {
+					s.error.Printf("ERROR (tarball.LayerFromOpener): %v", err)
+					return err
+				}
+				fimg, err := mutate.AppendLayers(empty.Image, l)
+				if err != nil {
+					s.error.Printf("ERROR (mutate.AppendLayers): %v", err)
+					return err
+				}
+				adds[i] = mutate.IndexAddendum{
+					Add:        fimg,
+					Descriptor: m,
+				}
+				return nil
 			})
 		}
+		if err := g.Wait(); err != nil {
+			serve.Error(w, err)
+			return
+		}
+		fidx := mutate.AppendManifests(empty.Index, adds...)
 		if err := serve.Index(w, r, fidx); err != nil {
 			s.error.Printf("ERROR (serve.Index): %v", err)
 			serve.Error(w, err)