Mobs with high levels of Speed begin to act strangely, such as pacing, running around in circles, or simply rotating.

This can be reproduced without using cheats by giving an evoker a Speed II splash potion whilst in a raid. On most mobs, however, it does not become noticeable until Speed III or higher. It is not to be confused with their normal raiding behavior without Speed.

Video: https://youtu.be/OzOTI7HTbOg