diff --git a/src/runtime/append_test.go b/src/runtime/append_test.go
index 6b8968e382..6bd8f3bd95 100644
--- a/src/runtime/append_test.go
+++ b/src/runtime/append_test.go
@@ -100,6 +100,22 @@ func BenchmarkAppendSlice(b *testing.B) {
 	}
 }
 
+var (
+	blackhole []byte
+)
+
+func BenchmarkAppendSliceLarge(b *testing.B) {
+	for _, length := range []int{1 << 10, 4 << 10, 16 << 10, 64 << 10, 256 << 10, 1024 << 10} {
+		y := make([]byte, length)
+		b.Run(fmt.Sprint(length, "Bytes"), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				blackhole = nil
+				blackhole = append(blackhole, y...)
+			}
+		})
+	}
+}
+
 func BenchmarkAppendStr(b *testing.B) {
 	for _, str := range []string{
 		"1",
diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index dd8dcb1873..7f4de450d2 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -111,19 +111,22 @@ func growslice(et *_type, old slice, cap int) slice {
 		}
 	}
 
-	var lenmem, capmem uintptr
+	var lenmem, newlenmem, capmem uintptr
 	const ptrSize = unsafe.Sizeof((*byte)(nil))
 	switch et.size {
 	case 1:
 		lenmem = uintptr(old.len)
+		newlenmem = uintptr(cap)
 		capmem = roundupsize(uintptr(newcap))
 		newcap = int(capmem)
 	case ptrSize:
 		lenmem = uintptr(old.len) * ptrSize
+		newlenmem = uintptr(cap) * ptrSize
 		capmem = roundupsize(uintptr(newcap) * ptrSize)
 		newcap = int(capmem / ptrSize)
 	default:
 		lenmem = uintptr(old.len) * et.size
+		newlenmem = uintptr(cap) * et.size
 		capmem = roundupsize(uintptr(newcap) * et.size)
 		newcap = int(capmem / et.size)
 	}
@@ -136,7 +139,9 @@ func growslice(et *_type, old slice, cap int) slice {
 	if et.kind&kindNoPointers != 0 {
 		p = mallocgc(capmem, nil, false)
 		memmove(p, old.array, lenmem)
-		memclr(add(p, lenmem), capmem-lenmem)
+		// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
+		// Only clear the part that will not be overwritten.
+		memclr(add(p, newlenmem), capmem-newlenmem)
 	} else {
 		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
 		p = mallocgc(capmem, et, true)