I found this thread on the steam forums. Not sure but after doing this my fps did went up with like 20 max. Didn't have any real low fps drops after this anymore (running TF2 at high settings). Check the thread and try it out, tell me if it worked.
sorry, longer ago: http://store.steampowered.com/news/?filter=client Steam client update released January 18, 2010, 5:01 pm - Valve - Steam Client Update *snip* - Optimized the stats and achievements system's disk usage Ofcourse its still used, so that might explain your case... I got however a good fast machine with win7 and dont notice any difference. on older machines mileage may vary then. Steam Cloud synchro in TF2 is cousing a lot of trouble atm... recommend to disable it.