[SDPhantom]

This isn't a FPS problem. The character isn't strafing either. He's far away in the center of the screen on a mount with a mob walking around him. If you look at the video, the mob animation is smooth enough while he's showing an example of what he's talking about.

In my observation while making my custom flight HUD, when you slightly turn using mouselook, there's a minimum delta of anywhere between 0.15° and 0.30°. When you have the camera zoomed out with an object near the camera as opposed to the focal point, this can be a large distance covered as shown in the video. Again, this is just how the game was made a decade ago.