Ball speed is derived 70% from your lower body and 30% from upper body. The higher ball position IMO makes it easier for the 30% portion to increase, but the lower body is the key for me. There are some simple things to do for this. For example, my ball speed went up considerably just by going to a more aggressive soul on my non sliding shoe, giving me the ability to push off harder and really drive into the slide.
Don't forget to talk to your coach, mentor or someone you trust with your game. Also, check out your local pro and see what they have to say...