no i think its purely a vsync / GL-buffer issue which each desktop environment seems to handle differently,KDE is using something called KWin and it handles the displaying of frames differently, in the settings menu (desktop effects) there are 3 - 4 different settings for vsync and they do seem to have a perceptible difference on input lag. I could get smooth graphics with some settings but it was like i was playing a pre-recorded version of what i had just inputted.. dropping from tripple buffer to double in L4D2 reduced it and removing vsync entirely seemed to give better results still.. so there is something about the frame buffer that is odd and not responding quick enough. From reading it seems like the archaic xserver (which is an OS within itself) is getting in the way.
Wayland and Mir hope to remove xserver and bring forward a frame perfect no lag display environment and they should be incoming in the next few rounds of distro releases
So all i can say for now until these new display servers properly arrive and get integrated is for you to experiment with KDE's more advanced vsync settings (on xfce i didnt see and easy gui for this) and from my experience feel an improvement through the smoother Kwin system which as far as i can tell is also being kept to integrate with wayland.
*you might have to use the opensource driver if you go with 4.11 KDE as there is a present supposedly solved bug but its still there for me which stops effects working on the desktop when using the latest AMD fglrx catalyst driver, 4.10 has no problem with this.
I know all about Wayland, and can't wait for it. It's just weird since I didn't have this problem before. It's a new thing. The delay is like looking at a stream on Twitch of your play while playing or something. I hope it gets fixed soon. After Wayland and Btrfs arrive, GNU/Linux is gonna be great! Except audio of course...