Happens more in creative mode.
When flying around and spawning mobs, it seems the mobs might have trouble despawning, I spawn a bunch of mobs, but then they reach the cap, so then I fly far away quickly, but the cap still remains full, Which I perceive as the result of mobs not despawning. It was specifically rabbits that were causing a lot of trouble.
When it happens the game tick seems to slow down.
Spawning a bunch of guardians had a similar slow down effect.