So I've been playing MC since late Alpha and never had this problem. A few days ago I played an hour or so, and got off. Just about 30 minutes later I hopped back on and my Minecraft was lagging horribly. I never got over ~10 fps. (My average FPS is around 200-350) This now happens every time I try to play SSP or SMP. I have downloaded and used Optifine, messed with the settings, and none of it is working for me. Please help. :]
This may sound stupid, but have you tried to restart your computer? Something may be doing something very resource demanding in the background. For example, a scheduled virus scan or an update.
Zombiecraft Maps!