If you're throwing properly from the slide step there should be little difference in velocity. Make sure that you're first movement isn't just straight to the plate so that you can be quick.
Think of the slide step as an abbreviatation of your regular leg kick. Lift your leg at the same angle as your normal kick, just lift it to a shorter height. When you lift your leg, bring it in toward your body a bit so that you get a bit of a "load" while your hips are moving toward the plate.
This will help you throw in a similar manner as your normal delivery. Since your lower half is moving quicker, your arm needs to move a bit quicker too.
Another way of throwing with the slide step is to stand with your feet shoulder width apart and almost click your front heel with your back heel and then step and throw. You will be plenty quick to the plate without sacrificing your power.
The problem a lot of pitchers have with the slide step is the mindset of being quick to the plate. Just understand that abbreviating the leg kick will take care of your time to the plate. You just have to worry about making it feel like a normal delivery. And the more you practice, the more comfortable it will become. Don't forget, most of our most important pitches take place with men on base, so practicing from the stretch/slide step may be more important than practicing from the wind up.