The slower the better, yes but, you also need to make sure you cycle your tank volume through the UV sterilizer as well. Personally I don't run them so I've stayed out of the thread until now. You'll need to figure out how many times per hour you want the volume of tank water to go through the sterilizer. Running 30gph through it will guarantee that whatever passes over it will die but the reproduction rates in the tank itself will far exceed what is being killed in the sterilizer. I'd probably shoot for a pump that can handle 5x the tank volume and use a valve to throttle it back some. Adjust as you see fit by the results you get.
Please note. I've read several times that unless you tear the sterilizer down weekly for cleanings, that the gunk that sticks to the glass will prevent the sterilizer from actually working. I forget the name of the one company that makes a sterilizer that comes with a wiper. You simply pull the plunger back forth on a weekly basis to clean the glass.