Approach speed is how I adjust ball speed. Faster for faster, slower for slower. If you can't adjust your timing for either you're going to have problems, though.
Tis the cheapest way of changing ball speed, though.
Current in-the-bag-at-random:
Track Robo Rule - OOB - 4.5 inch span
Black Widow - OOB - 4 inch span
Black Widow Sting - OOB - 4 inch span
Midnight Vibe - OOB - 4.25 inch span
Infinite One - OOB - 4 inch span
Complete NV - OOB - 4.25 inch span