It's all about hip angle. An open hip angle promotes breathing. A closed hip angle will engage the glutes and promote power. You play with hip angle two ways:

1) The more forward your saddle, the more open your hips.
2) The lower your bars, the more closed your hip angle.

Guys with very low bars for aerodynamics will tend to shift their saddle forward to open their hips and allow them to breath. Sprinters use a very closed hip angle to promote power generation, sacrificing some of their ability to breath in the process. Climbers and time trialists tend to have very open hip angles because they prize the ability to breath (aerobic power) over absolute power generation. Climbers will have a rather standard saddle setback and open their hips by raising their bars. Time trialists will have a very low bar for aerodynamics and open their hips by moving their saddle forward, often using a special frame with a steep seat tube angle to help this.

I don't think rpm is affected much; that's more about genetics and neuromuscular coordination. Some roadies conditioned to an open hip angle will find it hard to spin with a closed hip angle, but this is because their glutes, normally mostly ignored, are being asked to fire in a coordinated way and they just don't have the practice to do this. Track sprinters use a very closed hip angle and have no trouble exceeding 150rpm at full power.