If armspeed is what you want, then I'd advise experimenting with a Brinster hop and loosening up. The way you reachback is hindering several things as well. You are all crunched up at the reachback, so you can't get any centripetal pump from the ground. You want to go into compression during the throw. Also you reach too far back with the arm to close to the chest and lean too far back over the rear leg. When you lean so far back, you have move back forward with the body which slows your arm speed. You also get stuck weight back. When you reach back with the arm too far, your body gets in the way of the pull line.