I have a loop animation when you are holding a spear, but every time it restarts, it gets a little bit slower for a moment although I have timed the animation exactly so there is alway exact consitancy in the ovement speed. Here is a screencapture i made, to show this: - YouTube
Are you sure it’s a hickup? If you look at your animation curve, it is not a smooth transition.
You jump from a fast paced movement at the loop end, to a slow start at the beginning.
It looks that it might be the “hickup” problem.