Back when I was on my potato machine, I found that the adrenaline post process effect murdered my FPS. Even now, I notice a bit of a small drop when playing. So I found what was causing it and managed to suppress the effects such that the frame rate drop is minimal. I just need some testers to see if it works on other machines, and if it does, then I may as well share it to others. You can download the addon here. Simply put it into Steam\SteamApps\common\Left 4 Dead 2\left4dead2\addons folder. If it works or not, please leave a reply, thanks.