I also tried disabling triple buffering in my xorg configuration (Option "TripleBuffer" "False"), as I have that enabled normally, but it doesn't change anything.
I cannot use non-raw mouse input, as that makes the mouse in the game simply awful (lots of frame skips while looking around and there's some very weird accelaration going on; it's very much unplayable.)
I also play at the monitor's native resolution, in case someone things that the lag is introduced by the upscaling.
[s]Setting "fps_max 59" instead of 60 doesn't help either. With values over 60, the mouse lag is more severe though, so it does help somewhat, but not as much as under Windows.[/s] Nope, ignore that. Actually fps_max doesn't do anything.