It seems to be caused by the new friends list on the main menu:
http://www.teamfortress.tv/44103/is-tf2-mega-laggy-for-anyone-else-after-update

If you update your hud (all it involves is changing a 1 to a 2) it will get rid of the new friends list thing

instructions here:
http://www.teamfortress.tv/44102/custom-hud-not-working-after-update