I want to point out that I have uploaded an animation of this character before :p

The string is magic, but I did realize it just disappears once the ball hit the ground. I made it get sort of overdrawn and pull back towards the hand once the ball releases. I still feel like the ball needs one more frame after it hits the ground, but I couldn't come up with anything that looked right or didn't make it seem slower.
Hehe -- I was only kidding! ^__^
The "Imabadass" pose just made him feel so much like Sasuke (to me) with his "I'm too cool to be bothered, but I guess I'll do it and maybe you'll shut up." attitude he clearly exudes prior to the animation. That's not a bad thing though -- just the impression I got.

Anyways, regarding the ball --
I think the problem is that it appears to be generating some kind of resistance to the "string" in your new animation before it is suddenly pulled, but that "resistance" just comes out of nowhere. Perhaps you could show the magic "string" attaching itself to the ball somehow (and possibly causing the initial anticipation before the "pull" -- the "resistance" to the pull being from the string "hitting" the ball to attach itself to it -- that you're looking for?)
Regarding the "attack" portion of it --
The "release" of the ball (where the string detaches itself from the ball -- and possibly slaps the ground after the ball takes off into the air) is a key moment to emphasize the impact and strength of the attack that will be flying toward the opponent.
That being said, changing the arc of motion to better match the energy exerted is a good option.
The resulting angle of bounce is too shallow for the direction the ball is flying (in order to maintain its momentum), which is why the ball looks like it is "slowing down" to the eye (even with a small number of frames).
If you want the ball to look convincing, have it hit the ground (from behind the arm) at a more obtuse (open) angle (*before* it bounces) than what it is right now. Currently, the "bounce" angle is too acute (closed/tight) to retain the feeling of a fast-traveling billiard ball.
The number of frames isn't the issue -- it's the angle of bounce and the speed you're trying to reach *after* the bounce that is at odds. Yeah, it works in anime that things seem to move at whatever magical speed the artist wants it to, but it only "works" because the artist keeps the magical movements as close to the action as science allows. It's just that the magical forces anime characters exude sometimes come from otherworldly sources -- and despite how it appears, these forces always stay grounded in physical rules and the angles and the quantities of force/energy used/exerted always remain consistent across the board.
"Magic" is not an exception.