I noticed this in the go1.9 sources, though this also applies to go1.10. runtime.gobytes currently uses make([]byte, ...) and then immediately overwrites the data in the slice:

func gobytes(p *byte, n int) []byte {
	if n == 0 {
		return make([]byte, 0)
	}
	x := make([]byte, n)
	memmove(unsafe.Pointer(&x[0]), unsafe.Pointer(p), uintptr(n))
	return x
}

The zero initialization of the slice by make is a small but noticeable performance hit when copying a large chunk of memory via C.GoBytes. The allocation can be replaced with rawbyteslice(n) which already exists for the purpose of allocating an uninitialized []byte.