I don't know if you guys have played deluge, but in order to reload their weapons, their horses have to be going at a slow trot. That is the huge problem with horse xbow imo, they can run you over, while shooting you, and immediately reload before you even get up. Realism-wise, it makes perfect sense for the moderately difficult task of reloading a bolt on a moving horse require slow speeds, and balance-wise, you would fix all these sport-car xbowmen.