Gravity for slimes, guardians, and any type of entity except items and XP orbs doesn't work right. They move, stop, move, stop, move, stop, and it repeats, making slimes slower than anything. This also doesn't affect the player.
How to replicate:
1. Summon any mob in midair
2. Watch as it pauses every once in a while when it falls to the ground.

Unfortunately I can't take a screenshot as it requires video.