EDIT 2: Current official Steam client should have this issue fixed, its working for me. Doesnt seem like it works for everyone so will leave the dl link up

A lot of players seems to have problems with fps drop after the 8/3 steam client release.

Here is a temporary fix for it:

0: The newest official steam release should have the problem fixed. You can confirm the release version by clicking Help -> About steam, should display "Mar 10 2016" (or newer)

Proceed with this guide if it still doesnt work for you.