with online games, there are a lot of variables at work that can affect your fps. The amount of players on a server, the latency of the server etc. With that being said, to make sure there isn't something wrong with your hardware, would you post your temperatures for your CPU and GPU specifically? Especially while the game is running. If components get too hot, they will thermal throttle themselves to a lower speed to preserve safe temps. You would perceive this as a decrease in performance.