diff --git a/flatmap.go b/flatmap.go
index 1ce9c80..352176d 100644
--- a/flatmap.go
+++ b/flatmap.go
@@ -2,7 +2,8 @@ package underscore
 
 // Flatmap flatten the input slice element into the new slice. FlatMap maps every element with the help of a mapper function, then flattens the input slice element into the new slice.
 func Flatmap[T any](values []T, mapper func(n T) []T) []T {
-	res := make([]T, 0)
+	// Estimate capacity: assume average of 2-3 items per element
+	res := make([]T, 0, len(values)*2)
 	for _, v := range values {
 		vs := mapper(v)
 		res = append(res, vs...)
diff --git a/flatmap_test.go b/flatmap_test.go
index ff5ca41..714c0b3 100644
--- a/flatmap_test.go
+++ b/flatmap_test.go
@@ -15,3 +15,16 @@ func TestFlatmap(t *testing.T) {
 
 	assert.Equal(t, want, u.Flatmap(nums, transform))
 }
+
+func BenchmarkFlatmap(b *testing.B) {
+	data := make([]int, 100)
+	for i := range data {
+		data[i] = i
+	}
+	mapper := func(n int) []int { return []int{n, n * 2, n * 3} }
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.Flatmap(data, mapper)
+	}
+}