Superflat Slimeblock World is causing "Already Tesselating!" crash / Major lag when using a big amount of Slime Blocks

I created a super flat world that had only 50 layers of Slime Blocks and from the second I pressed "Create World" I noticed it was taking a long time to load the world. Even in the world I was down to about 1-5 FPS instead of my usual 30.

Pastbin of CrashLog: http://pastebin.com/1hHWV9hH