Hey Mozzy,
I think the main issue is that you have a lot of frames after the motion blur, moving the whip just a little bit. This makes it look like the whip slows down for no reason. Here I removed frames 4, 5, and 6, and a bunch of the frames after the whip. I also modified the motion blur so there was more of it at the tip then the base. I think it gives the whip more pop:
The other issue is what is going on after the whip hits. It looks like he is sort of moving it up, then down and back, but it is not very readable. I would add one motion after the actual whip crack, where he is pulling the whip back to the idle position, and probably wouldn't motion blur it.
Hope this helps a little.