If two identical arrows are fired into a hay bale but one of those arrows is traveling twice as fast as the other the arrow traveling twice as fast will penetrate four times as fast as the slower arrow will. This same principle is one of the reasons that guns are so deadly.
If a bullet did not have the speed that it has when being expelled from a gun it would be pretty harmless in most situations. You could manually throw a bullet at someone and it would not be deadly at all. It wouldn’t even hurt if it was not close range. The faster the arrow the more damage it could do.