On a handgun with a compensator, does the compensator work best with slow powder loads or fast powder loads? And why?

Not sure of the answer to your question, but I suspect that the effect is negligible. Bullet weight is another matter, however. At the height of gamesmansship in IPSC, Enos, Jarrett et. al. were playing with the various factors and came up with very light bullets and efficient comps. yielding negative muzzle flip ie. the muzzles went down! Now these were with very light bullets (of the order of 90-100 grains) and very high velocities (of the order of 2000 fps). I addition, not all comps. are created equal. There is a comp for rifles that also seems to yield muzzle drop rather than muzzle rise!
Peter.

We have found that the higher the gas pressure coming out of the comp the more efficient they are. We went from one port up to three-four and five. The 3 port seems to work as well as the four and five. The angled surfaces of the comp allow the gas to exert more downward pressure and we get less muzzle rise.

I shoot a 9X21 Para with a Nowlin type 3 port comp. It makes a huge difference shot to shot.

I have a Watson style with two cut ports on the sides of teh chamber that does work OK but not as well as the 9X21. The Watson is on a 45 and, of course, has less gas pressure to work with.

Greg

Light bullets, Fast powder regardless of the amount of ports, all other factors being equal

Lighter bullets and slower powders usually mean a flatter gun (compared to non-comped guns). My Bedell STI is flatter with N105 than with faster powders. I use 124gr because IPSC have min wieght of 120 for major. USPSA don't and some use 115gr. But not many use lighter than that.

Downside with slower powders is bigger blast and possibly more spilling while loading due to full cases.

But the bottom line is that one has to try the different combinations and see what works best for you. Guns with a lot of ports might need slower powders to work, while other use a little faster.