Extreme frame rate drop when playing in snapshots, almost unplayable.

Put the summary of the bug you're having here
When entering "More World Options" or just playing a Minecraft world in snapshots, I get about 1 frame per 2 seconds.
What I expected to happen was...:
Describe what you thought should happen here
No lag, I usually get at least 60 fps.
What actually happened was...:
Describe what happened here
Extreme lag.
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...Make a new world
3 Enter More World Options
OR
1 Play existing world