If you've watched tennis on TV, the balls come up high they hardly do the offensive driving forehand slice. The only forehand slices you see is when they're on the run trying to get it back -- A defensive forehand slice. They take the forehand across the body when it does come up that height. A safer shot. It's a hard shot to control, and most of the time will land in the net or way out.
http://tennis.about.com/library/media/fh-slice.WMV
That's the forehand slice drive -- It's the same exact one I use. Go outside right now, bounce a ball up and hit it like that. Or evne throw it up and hit like that. You'll see for yourself you can forehand drive slice pretty fast.
Mine is just inside the court (not at the baseline), and obviously swung a lot harder than that video. I'll see if I can borrow my friends kodak digital camera to take video and setup a session with my coach after the boys tennis team practice either monday or tuesday.
You can generate a lot more racquet speed going downwards, than sideways and forewards. I don't see many pitchers pitching sideways, a lot of them go from high to low on their pitches for obvious reasons.