Just a stab in the dark but how's your latency doing? Might be hitting some lag from ISP or the server. With the amount of activity those servers are getting right now I can imagine their loads are huge. I'm sitting in a 500 queue right now and the game isn't officially out.

The way you describe it- seems the more and load the servers got the worse FPS you got. Once again just a stab in the dark really.