When I was doing raids I noticed that after a while into it the mobs would start stuttering and not moving and then move really quickly. The longer the raid went on for the worse it got. I eventually figured out it is caused by the vexes, evokers are spawning way too many vexes to begin with being caused by MC-147574. After there are about 9-12 vexes it becomes noticable (and since I am doing level 5 raids on hard difficulty that is reached very quickly) and after about 21 vexes are spawned (which is still a very reasonable amount being that there can be 3 evokers at a time and each spawns 3 vexes at time), the lag becomes so absurd that the mobs keep moving and freezing. The FPS is fine while this occurs, it only seems to affect the server.

I was in survival mode when this occurred, so I don't know if it's just the fact the vexes exist or whether it is caused by them tracking you

In the screenshot the fps looks super bad but it's just because there were so many entities on the screen, plus the debug screen lags the game too much as well.