Mobs lag or have choppy animation; does not affect the player model

While using the snapshots, I noticed that every mob moves very slowly, with choppy animations and the like (for example, when a slime jumps, it snaps to a position in the air, hovers for a moment, then zooms back to the ground). However, this does not happen in the official releases; this only happens in recent snapshots after 1.7. It doesn't seem to be game lag because the player model isn't affected and FPS has no clear influence. The game can run at 60 FPS, but the mobs act as if the game was running at 5. Thanks for any support on the issue!