Not sure if this is a bug or not.
Essentially there is a folder that steam continually writes to when you play games through steam and when you have an active AV that checks files, the activity of the AV software slows down the performance of games. I tried this out and it appears to be the case. I added the specified folder to the AV exclusions list and I gained a noticeable performance increase in most games played through steam (not just TF2).
This is just what I found.