It's not "kernel is slow", kernel when left to its own devices is plenty fast, the reason is that when you want to make decision about packet in userspace (vs telling kernel what to do with it via various interfaces) that kernel logic would just be overhead.
It's similar for applications; if you can, say, decode whole DNS packet in one go, you don't really want kernel to spend time decoding UDP packet, then you decoding the rest of the packet; doing it in one step is much faster.
