I am experiencing a very long lag while typing at my normal speed, also the site responds very slowly to things like mouse over and clicks. I am on FF 3.5 and puppy linux (it used to work with the same config with the design on chiphacker)
I used to experience this all the time with the original StackOverflow. Now I experience it with everything! Spinning the mouse wheel doesn't scroll the page until when it feels like scrolling. Slow key-to-visible text action, including the backspace. Just sliding the cursor from one window to another = no action until 10-20 sec later (I use "focus follows mouse" on icewm on a Linux machine)
Other people experience this, but many more with the same setup, same software versions, do not. It's a known problem, but only among us insiders - like with UFO witnesses, outsiders just don't "believe"! I've tried obtaining stack dumps using oprofile, gdb, etc but it's not clear what's going on.
Solution: 1) use Opera or something else, not FF 2) quit FF, restart. This doesn't always do it, but quitting my X11 session and restarting always works. A true solution lies in fixing whatever bad interaction is going on with FF, X, the kernel, and threading.
I had a similar problem, and it was one of the firefox plugins that was periodically taking over the process to poll some site looking for updates.
I knew it wasn't the computer (8 core, 8 gig) or network (T3 here at work).
I deleted almost all my plugins and now everything is back to being snappy. I'm not sure which one was the culprit.
I think you're just seeing the bug that makes my life miserable every day, bug 646941.
Restarting Firefox usually helps, but it comes back eventually.
See also bug 490122, which describes the general symptom.
(if that is what you're seeing then it obviously has nothing to do with StackExchange; it's a Firefox problem)