Check the system requirements of the games you want to play, is your system at least as good as their recommended system requirements or does it barely meet the minimum system requirements? If your system barely meets the minimum requirements make sure the graphics are on the lowest possible settings.
If your problem is FPS this can be improved by not having any other programs running the background (except of course your AV software) and no other windows open. A good disk defragment can also improve your computer's performance a little or a lot depending on how badly fragmented it was to begin with.
Some people confuse input lag with FPS if your problem is actually input lag then the issue is more likely your speed of broadband connection. Talk to your ISP about it.
If your computer is easily superior to their recommended system requirements and you have defragged your HDD and have nothing else running in the background and high speed broadband and it's still lagging the problem may be their end not yours.