Help me, internets :(

So I tried playing Minecraft on my PC (knowing it's going to bother my arm, but I should be okay if I don't do too much at a time), and ... it's unbearably laggy. I mean, the first thing I tried was a multiplayer server, so I thought it was just that, but then I tried singleplayer and it's still unplayable.

Things I have done:

* Turned all video settings down to craptastic-but-fast
* Deleted Minecraft and did a clean reinstall
* Run without mods (since I hadn't gotten around to installing any)
* Made sure java was up to date
* Ran a virus scan
* Rebooted my computer

It still lags terribly, even looking around from the spawn point of a new Peaceful world.

Halp?