User blog comment:Gundam Legilis/RWBY Vol 3: Enter the Dillon/@comment-10380701-20150728033337/@comment-10380701-20150729144036

Skipping a frame is theoretically impossible in model animation, due to keyframes. Unless the model is forced to load an animation, then that delay is likely the model loading the animation.

Alternately, the in-between in two keyframes have such a freakishly short time that the model looks like it skipped a frame entirely.

That said, I don't know what MMDs you're watching, but the well-made ones are nearly on par and (IMO), neatly surpasses RWBY's animation quality. But sturgeon's law applies and 80% are pretty shite.