Mine was low flow until I added the powerhead. It is still a little less than medium though. Okay, it is still low flow.
The way the plumbing runs to the refugium, the water flows on a slight decline straight to the skimmer section. Any water that falls down the Tee is what goes into the refugium, and the gate valve I installed ended up being pointless.
In this very cluttered shot, the red handle is the gate valve.
Water tends to follow the path of least resistance, so it moves straight far more easily than dropping down into the refugium.
When I had my 55g running, the refugium was 5g in size and the flow through it was around 45 to 55gph. On the 29g, the sump was mostly refugium, and the return pump moved about 300gph so that was how much flow moved through a 14g volume.
I have no idea how many gallons move through the refugium under my reef, but it is very steady. I guess next time I do a water change, I'll pump water out of the refugium and then time how long it takes to refill x amount of inches to figure out the current flow / refill rate.