I agree that you want the fuge to receive clean water, isnt that why the skimmer is in first chamber ?
And a sock if you have excess particulate ?
I feel you can have slower flow through your fuge, as we have done. We run the 5000lt/hr with two T-off to UV and Reactors so technically I run probable about 4000lt/hr through mine. It works just fine ?
Are you saying I need to increase by return pump flow to the DT ?
Yes, you want to give the refugium time to process the water your running through it.
Flow through the entire sump is a little different. In the early days of reef keeping the "magic number" was that your wanted to turn over the tank volume 5 times an hour or more. Since them it's been going up and people tend to recommend 10 or even a lot more. People seem to demand more and more flow through the filtration system.
I'd say that in many cases you do want high flow, but it's need not come all from the filtration system. You can add a closed loop or circulation pumps to the display tank, and get a lot more flow from that, rather than make the entire filtration system larger.
In this day and age, your also trying to keep the cost of running the system down. It's often much less expensive to run a circulation pump compared to a return pump. It's also less expensive if you can run a smaller return pump. It all comes down to striking a balance between everything.
To give you an idea, at one time I was using an Iwaki WMD-100RT pump to run everything. Lots of flow, high pressure, and massive power consumption. The electric company loved that. When I upgraded the filtration system I was able to replace it with two smaller pumps and the system consumed far less power.
The bottom line is that if your livestock is doing well and your getting a turn over through the filtration system of roughly 5 times an hour and you have enough additional circulation in the display tank, you should be fine.