I'd bet you had a browser running for a long time, probably with loads of windows open, right? I've seen the lag which you describe, and it is always caused by thrashing (swapping itself senseless). next time you get that kind of lag run "free" in an xterm and you'll see you're likely around 200mb into swap. The browser is a hog, kill it when you aren't actively using it and your experience will dramatically improve.