Best case scenario: A bad driver update or some new software is causing the lag. Firstly try to reinstall your video card drivers, then roll back if that doesn't work. Also check to see what software you installed recently, and go through all the programs that launch on start-up. Try and disable any unnecessary programs from launching unless you need them.
Also check your addons for your web-browser; since Battle-Log will open your browser, an addon you have might be slowing down the system.
Worst case scenario, if you're comfortable with it, open up your case for a bit and just check to see if there's any visible signs of overheat. You might have damaged your card. I doubt it, but it's good to check and be sure.
