So, I don't remember how I got it sometime back in 2015, but it seems I have just over 200 shots of Ball Speed Boost Plus (3x) on my account although I can't seem to use it.
When I start a round, it shows up as active in the game options as well as in icon form at the top of the screen. However, when I hit the first shot, there is no speed boost. From the second shot on, the icon disappears and the boost is not selectable in the options menu.
When I finish the round and go back to the main menu, the boost shows up. Start a new round. Rinse and repeat as before.