You should find out the pump size. That said, no, you don't need slower flow in the sump. You have a skimmer in there you must feed water to. Your return pump should at least match the flow of the skimmer pump, and any other reactors in there. It also needs to match the overflow box. You need to find out what the gph
is on that and match it. Otherwise air bubbles may accumulate in the J tube and eventually cause a siphon break.
That sump is a wet/dry, and that rock should not be above water. You are losing it's denitrifying ability.