Expected:
Entities move at normal speeds and minecraft performs fine in a superflat world with just a level of barrier blocks.

Found:
Everything in that barrier superflat world moves at a slower pace, making it frustrating to play. Entities even fall at very slow rates.

Replication:
1. Create a superflat world with the gen set as 3;minecraft:barrier;1;
2. Spawn slimes and observe as they slowly jump around.

Here is a video proving this bug:
https://www.youtube.com/watch?v=kOxq3RhIOyw