1) Don't step through. Just dismount with your right foot behind, it's not any slower or less safe, and it's what the vast majority of elite racers do.
2) Test whether you prefer having your left foot clipped in until the last moment, or unclipping early and resting it on top of the pedal. Personally, I prefer staying clipped in.
3) To get the dismount as fast as possible, you have to move your right hand to the top tube and support a good part of your weight on that right arm before the dismount.
4) Practice, practice, practice. Start at a crawl, then ever so gradually speed up. Don't speed up until you have it nailed slow.