Too jumpy, not too slow. Adjust and smear that between frame and have the end go a bit slower as the Power of the swing is diffused.
Alternatively, for a more Berzerk e chunky look you could have the slash stop at that lowest point at the end where it's fully stretched out like he's done a big chop and then have him drag it back. Bonus points for embedding in the earth a little!
EDIT: rephrasing since I posted this on mobile:
Have the swing be the same speed at first, but slam into the ground on that last frame. It'll lend it a lot of power. Then for the reset have them drag the sword straight back towards them into the ready position. Most of the animation here should be in that recovery, since putting too much in that leading bit will undercut both the required-by-the-game immediacy of the attack and the TIMING of the attack.