I start by spreading the polish fairly quickly at a low speed, then as it begins to get to a working stage, I crank the PC up to 5 or 6 and start doing slow passes, alternating vertically/horizontally. It's been working for me.
When I first started using the PC, I was moving it much faster than I do now, and had less than stellar results. So...at least for me, fast passes didn't work. I just don't think it gives the polish time to actually polish a particular area of paint if you pass over it too quickly. Just my unscientific thoughts.