transform.Crop performance goes bad when rect size is small

[Sora233]

my result is below.

goos: linux
goarch: amd64
pkg: test
cpu: AMD Ryzen 7 3700X 8-Core Processor
BenchmarkMyCrop-16                 14307             79645 ns/op
BenchmarkResizeCrop-16               134           8771603 ns/op

my test code is below.

const size = 50

func BenchmarkMyCrop(b *testing.B) {
    f, err := ioutil.ReadFile("img.png")
    if err != nil {
        panic(err)
    }
    dImage, _, err := image.Decode(bytes.NewReader(f))
    if err != nil {
        panic(err)
    }
    rect := image.Rect(0, 0, size, size)

    SubImage := func (img image.Image, r image.Rectangle) image.Image {
        var bgSize = image.Rect(0, 0, r.Dx(), r.Dy())
        var bg = image.NewNRGBA(bgSize)
        draw.Draw(bg, bg.Bounds(), img, r.Min, draw.Src)
        return bg
    }
    for i := 0; i < b.N; i++ {
        SubImage(dImage, rect)
    }
}

func BenchmarkResizeCrop(b *testing.B) {
    f, err := ioutil.ReadFile("img.png")
    if err != nil {
        panic(err)
    }
    dImage, _, err := image.Decode(bytes.NewReader(f))
    if err != nil {
        panic(err)
    }
    rect := image.Rect(0, 0, size, size)
    for i := 0; i < b.N; i++ {
        transform.Crop(dImage, rect)
    }
}

my test image is below.