PID73 mentioned this first (at least that I saw) in a thread about tree movements.
I'll pick up from those comments by PID73 here on their own thread.
The fast scanning that our characters do while they are inactive is funny, and tends to give a little flavor of paranoia which is fine in a post apocalyptic world, however it really isn't needed.
If you ever watch someone who actually knows how to move around in the wild and be aware of what is around them, they actually do their visual and audio scanning with slow movements of the head, and heavy usage of peripheral vision and hearing to detect movement.
The quick snap of the head motion is basically only when a fast moving target must be quickly locked up in binocular vision for accurate depth perception.
Quick, jerky motions actually reduce your ability to be inconspicuous. They also tend to make me think my character is a meth-head, lol.
I don't know if Xsyon could get any performance enhancements from significantly slowing idle character movements, but if so, there's more than just a performance reason to do so.