Why does having gamepad plugged in cause stuttering in l4d2.

[Floid]

As title really. It's a standard xbox gamepad with usb for PC. I use it for stuff like dark souls and most 3rd person games like lords of the fallen, AC black flag etc. But if it's left plugged in while playing l4d2, it always causes stuttering while in game. I have to exit, unplug the gamepad, then restart l4d2, to get rid of the stuttering.