We seem to have two possibilities here. One, your slowdown is natural, and you just aren't built for a fast delivery. In this case I would advise that you use a slide-step all the time, otherwise you're wide open to the stolen base, because runners will take full advantage of a pitcher who is slow to the plate. Also, in this case you would need to work on pickoff moves.
The other possibility is that you want to increase the speed of your delivery, and that's something you can work on. I'm not talking about the "quick pitch", because that's illegal. I'm talking about deliver the pitch, get the ball back and right away go into the windup and deliver the next pitch. Ever see Mark Buehrle pitch? He's a prime example of working fast, and you would do well to study him, see how he does it, and try to emulate some of his moves.
That's about all I can tell you. When I pitched, I worked fast and used a slide-step all the time, because it was convenient and comfortable for me and it enabled me to stay focused on the strike zone. Some of the other guys on this forum might also have some ideas, so watch for additional tips. 8)