diff --git a/internal/codec/pdf/color.go b/internal/codec/pdf/color.go
index 4059e31..609771c 100644
--- a/internal/codec/pdf/color.go
+++ b/internal/codec/pdf/color.go
@@ -250,6 +250,9 @@ func CompositeGroupRegion(ctx context.Context, pixels, initial, result []Composi
 	}
 	components, parentComponents := space.Components(), parent.Components()
 	var target pdfgo.ColorantPixel
+	var previous, previousInitial, previousResult, composed CompositePixel
+	var previousAlpha float64
+	ready := false
 	for y := region.Min.Y; y < region.Max.Y; y++ {
 		if err := ctx.Err(); err != nil {
 			return err
@@ -265,6 +268,11 @@ func CompositeGroupRegion(ctx context.Context, pixels, initial, result []Composi
 				alpha *= mask[i]
 			}
 			pixel := &result[i]
+			if ready && pixels[i] == previous && initial[i] == previousInitial && *pixel == previousResult && alpha == previousAlpha {
+				pixels[i] = composed
+				continue
+			}
+			previous, previousInitial, previousResult, previousAlpha = pixels[i], initial[i], *pixel, alpha
 			target.Values = pixels[i].Values[:parentComponents]
 			target.Alpha = pixels[i].Alpha
 			target.Shape = pixels[i].Shape
@@ -277,6 +285,7 @@ func CompositeGroupRegion(ctx context.Context, pixels, initial, result []Composi
 				return err
 			}
 			pixels[i].Alpha, pixels[i].Effect, pixels[i].Shape = target.Alpha, target.Effect, target.Shape
+			composed, ready = pixels[i], true
 		}
 	}
 	return nil
diff --git a/internal/webui/wasm.go b/internal/webui/wasm.go
index 994d380..1635d35 100644
--- a/internal/webui/wasm.go
+++ b/internal/webui/wasm.go
@@ -132,6 +132,7 @@ type outputContext struct {
 	cancel     context.CancelFunc
 	checkpoint func() error
 	checked    time.Time
+	pending    uint8
 }
 
 // Write 将数据块交给浏览器保存
@@ -5490,6 +5491,11 @@ func (c *outputContext) Err() error {
 	if err := c.Context.Err(); err != nil {
 		return err
 	}
+	if c.pending != 0 {
+		c.pending--
+		return nil
+	}
+	c.pending = 15
 	if time.Since(c.checked) >= 50*time.Millisecond {
 		c.checked = time.Now()
 		if err := c.checkpoint(); err != nil {