Gmod Lag

I Have a gtx 550 ti and a amd fx 8120 oc to 4ghz but when i play single player gmod my fps is in the 300’s and when i join a multiplayer game my fps will automatically drop into the 30’s this should not be happening with this graphics card. I can change my settings to the lowest but the fps still wont change. I Know for a fact that this game can run better on my pc.

plzz plzz plzz help