I have found that with diminishing abrasive polishes like DACP, Poorboys, etc. that a speed of 5-6 works best because it generates enough friction to allow the abrasives to break down properly. I have found that running it too slow with the more abrasive products will sometimes cause slight scratches because the product does not break down.
For Sealant application (UPP, NXT, etc.) I use a speed of 3-4.
This may change for you based on how much pressure you apply and how much product you apply to the pad.