one thing not discussed here yet is the quiver size....
a narrow tube quiver dictates that the arrows must be pulled straight (or almost straight0 out the end of the quiver, and most quivers positioned high on the back make it very difficult to draw the arrow by the nock without running out of arm length. So guys grab the shaft by the feather area and this slows down the knocking system.
a larger Hill style quiver that lays diagonally across the back allow the arrow to be pulled by the nock, and the arrow is removed diagonally from the quiver, meaning lower arm angle, enough arm length to fully remove the arrow by the nock, and easier/faster nocking of the arrow on the string.
flexibility isn't really too much of a problem with a loose-fitting Hill style quiver as the quiver bottom is bumped up into position each time you grab an arrow....you can bump the quiver as high as necessary to reach the arrows, and then the quiver falls back out of the way so you can go through brush....if you look at old photos of archers using a Hill style quiver, including Hill himself, you will notice that the arrows are hanging off the shoulder, not up by the ear....target/field archers using backquivers have the arrows up by the ear...