If you checked descr_skeletons.txt and made an excel comparison of each skeleton (like I did hehehe) then you'd see that the only thing that's different in animation between fs_slow_whatever and fs_whatever is the running animation. That is it. One single animation. Just like the fast and semi-fast ones. They walk the same, but run at different speeds.

So it might be something to do with the skeleton and not the animation.

I have noticed one thing with fs_slow_whatever: when a unit is routing and getting killed, the model shrunk. Yes, you heard that right, it shrinks. It's like the skeleton is collapsing inward. It changes back to normal for the "falling on ground dying" part of the animation though. But the death-hit animation, they do the weird inward shrinking thing.

My conclusion is fs_slow_whatever is inherently flawed. CA probably used a newer CAS to export the new animation speeds and they're not agreeing with the older ones or the engine. So that might be affecting their kill-ratio.