When shooting pentax 6x7 hand-held I noticed that the view angle affects the sharpness at low speeds. If I use the WLF I get better sharpness than with the prism; shooting with prism straight ahead is less sharp than shooting with the camera tilted downward. So I guess it depends, among other things, on how stable your stance is at the moment of releasing the shutter. I remember in pistol shooting training that I went through many years ago, that the first step in training was to find the right stance, which should be relaxed and stable. Once you find that stance the shooting accuracy improves considerably. Maybe it's the same with shooting MF and maybe that's why you find people report getting sharp results with very slow speeds while others struggle at higher speeds; the former just found their correct stance.