Just for the sake of eliminating all possible scenarios, double check that your monitor is plugged straight into the GPU and not into the motherboard. If it was plugged into the motherboard your PC would be trying to run the game from your integrated graphics, which would explain the low fps.
Also, type the command 'perfoverlay.drawfps 1' (without coma's) after pressing the ~ key when the game is loading up, or if you are already in game. This will show your fps, and you could determine whether your problem is hardware or network related.
Good luck.