This is really all you're going to be able to do to slow people down who are quick pitching you. I do the practice swing thing when I get down to 2 strikes, in the hope that the pitcher will be impatient and still pitch the ball even if he can't see the reticle. Sometimes he misses his spot and I get a good pitch to hit.
Now if he's using a slide step, I don't know about you, but I always lose a lot of control when I slide step. I usually end up hanging my pitches, and I would assume most people would too, so you should be getting something to hit.