Minecraft lagged out on me(which it never does) when creating a costimized superflat world when editing the superflat preset to be different blocks

Minecraft lagged out on me(which it never does) when creating a customized superflat world when editing the superflat preset to be different blocks.

*What I expected to happen was minecraft to run normaly at about 25-30 fps

*What actually happened was when I started up my custiomized superfalt it started lagging out to 0 fps. I tried restarting minecraft multiple times and no changes.

Steps to Reproduce:
1. Get Minecraft Pre-release 1.4.2 or 1.4.1.
2. Make a superflat world customizing the preset with different blocks.
3. Minecraft will start lagging out at about 0-2 fps