The fix for this (and input lag in general, even in other games) is to run the game with libstrangle, like this: (pick whatever target framerate you want)
strangle 60 mono ./Vintagestory.exe
I am not sure why this helps in Vintage Story, as usually this "trick" only solves the 1-2 frames (at minimum) worth of input lag in other games (such as most Source engine games) by changing the moment at which the framebuffer is flipped enough to stop extra frames from being buffered in the GPU driver side (nVidia drivers for example are known to buffer anywhere from 1 to more than 5 frames).
This however, does not explain the 0.5 - 1s worth of lag in Vintage Story, as that is very excessive. Perhaps OpenTK (if that is still used for windowing and input) has a sub-optimal input handler which makes things worse on high input rate (1000Hz) devices and libstrangle somehow manages to "throttle" that as well, so it has to process less events (and less stale events) each frame.
In any case, with this "tweak", the input lag is almost completely gone, and moving around no longer correlates to higher CPU usage as well.
