Your thumb is in the ball way too long to have a high rev rate. The quicker you can get the thumb out and rotate your hand around the ball(up the back, on the side ect) with the fingers still in the ball, the better your rev rate will be.
Get a very light weight house ball, like 6 lbs or maybe 8, and throw a few without your thumb in the ball at all to get an idea of what it should feel like.