When I load any world in 15w42a it goes through the usual 'downloading terrain' stuff then after ~3-5 seconds it crashes. In the launcher tab 'Game Output' it says "[Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3535ms behind, skipping 70 tick(s)". This happens 8 times in 10 with the other 2 times throwing up a java error which saved an error report in the .minecraft folder (attached). I've tried loading up older worlds but it still crashes.
What I expected to happen was...:
the game to load

What actually happened was...:
it crashed

Steps to Reproduce:
1. launch minecraft
2. create/load world
3. wait
4. crash