Entered Nether, now 2fps & crashed.

Put the summary of the bug you're having here

What I expected to happen was...:
To enter the nether with normal FPS... (should be capped at 30)

What actually happened was...:
Entered nether, has 2 golums, pigmen, and a couple of zombies next to the portal. FPS was about 2-5 and figured it was just due to loading the level. Attacked baby zombie that was wearing full golden armour; I attacked with a flame & knockback (highest) sword. The baby Zombie, now on fire, slid backward on the ground for about 10 blocks (now at 0-2 fps) and the game crashed. (sad face)