When I went to play a world I made last week in 20w20a, I was met with severe lag when it came to mobs and player actions. Player movement was still smooth, but other things were laggy. Mobs and other entities were very choppy, elytra deactivates much slower, and applying effects such as night vision has extreme delay. There could be other things that I missed, but overall severe lag in this version. This also happened in 20w20a, but not in 20w19a.