I usually get 30 to 75 FPS, but after launching a firework out of render sideways from a TNT explosion, I never saw more than .05 FPS and had to restart Minecraft. Afterwards there was no lag after I did the same thing.
Environment
Win7 64 bit OS, 2.8 GHz dual core Pentium D, 4GB 400 MHz RAM. 2gb VRAM, Java Version 7 64 bit
This ticket has been marked as a duplicate of MC-5120 , in favor of the better description/information provided.