In the Arma3.cfg file there's GPU_MaxFramesAhead and GPU_DetectedFramesAhead.
Now, I'm not sure if it's one of them or both of them, but when I set both to 0 it makes the game feel far more responsive.
It would be very helpful for those with mouselag problems to have these settings (or one of them) available straight from the graphics menu, especially those who are a bit shy about messing with cfg files.