one thing that really puts speed on my ball is not grabbing it at the bottom. for example, my normal strike shot is about 16 MPH. but, when i take out all my hit for my spares, my speed jumps up to 20 MPH-without me even trying to throw the ball harder. it just happens naturally.
now, that 20 MPH is also accompanied by nearly zero rotation. so if i want to play straight, i kill just partial hand, and my speed is around 18 MPH. but, i still maintain decent rotation.
these speed increases happen completely on their own-i feel zero difference whatsoever, except for less hit at the bottom. just remember, grab less at the release, and try to exit the ball as cleanly and quickly as possible.
i hope this helps, and good luck!