I'm making an FPS and very frequently the mouse look stutters, making the game very unpleasant to playtest. Also, when any mouse button is pressed the camera loses "precision" (i don't really know how to describe it) and snaps very harshly. I'm using kubuntu 18.04, If that helps.
Having the exact same problem over here, this happens with the Standard Package FPSController. On Unity 2017.x it feels more like a huge amount of input lag, and Unity 2018-9.x it is as RequiemForMayo described huge spikes of stuttering. Pressing any mouse button also makes the problem worse on both versions. Tested on ElementaryOS 5, and Pop_Os 18.10.