For the last couple of days, I've been getting about 20 FPS at best, no matter where I am in WoW.

Previously, with the exact same set-up, I'd been getting 60 FPS consistently - smooth as butter.

Was there an update of some kind in the game or something that's caused problems?

I've tried updating drivers, disabling add-ons, loading one character at a time, etc. etc. with no luck. No other games on my system have a similar problem, and I haven't noticed any performance decreases anywhere else. Setting everything to the lowest settings does nothing, either. I'm at 20 FPS whether my settings are set to max or set to minimum.

Hardware:
6850k cpu
64GB RAM
2x 1080

I've been using this machine for about 6 months with no issues.

Any suggestions? I'm stumped.