I would say fast because you would have more water movement to stir up the detritus. However,If your talking turnover thru a sump, I would want the rate to be slower so the water has more time to react to with media and skimmer
10X tank turnover per hour. That's why setting up a true fuge separate from your sump is the way to go if possible. We are forced to compromise with a sump/fuge combo. A fuge is ideally supposed to have slow flow. That way delicate pods can thrive but the flow forces some in the DT and the pod population sustains itself AND feeds the tank.
I agree with the folks up top. I have a fairly fast flow in the tank, but I have a custom sump made by Jeff that has a separate chamber for my fuge that is pumped by a small pump, and it empties back into my return slowly. I added some Tigger pods 4 weeks ago, and all of the sudden they are everywhere.