I'm not sure if this is an intended feature or a bug, but after installing Requiem, I noticed that mobs, specifically hostile mobs, move much much quicker than the usual pace.

I'd really like to get more information on this issue, because there is nothing in Requiem that could speed up unpossessed mobs.

Since I don't have anything to go off of to fix this bug, I'll go ahead and close this issue. Feel free to reopen it if you have more details.

Actually I think I may have experienced this at one point.
It specifically happened after I set my soul state to false using commands while possessing a skeleton.
The skeleton moved much faster than I thought it would. Just might be me seeing things though.

Ah, and they keep being faster even when not possessed ?

You may want to issue the following command and then post the resulting log output:
/execute as @e run attribute @s generic.movement_speed get