This has worked for plenty of people on SPUF (copied from there) so I suggest you read it completely, however be aware that your millage may vary, perhaps you will gain something perhaps not, but you will never lose

Everybody has experienced how FPS are going down the drain recently with TF2, however I believe I've found one of the sources (I'm pretty sure there many others) and found a way to fix it, so enough with the rambling I'm going straight to the point:

The folder:
C:\Program Files\Steam\appcache\stats

Contains a file called:
UserGameStats_####_#### (# = some numbers)

This file keeps records of god knows what about TF2, and it gets updated EXTREMELY frequently while playing.

The problem is that, most of us run high quality Anti-virus programs and they tend to scan all files that get updated in the system, and a lot of them tend to do so the instant it happens, clogging up the processor and making our FPSs die.

Apparently, having a multicore processor and fast hard drives (or a RAID 0 config) makes this even worse.


HOW TO FIX Video (credits to MafiaGuy7)

Open up your Anti-virus program
Go to the settings menu
Select Exceptions/Exclusions
Add the following folder to the scan exceptions
C:\Program Files\Steam\appcache\stats

Reboot if your Anti-virus needs to and enjoy a smooth game of TF2
NOTE: User who have performed this action have been reporting frame-rate gains or stability in other STEAM games, while it seems plausible I have not tested it myself.