OK - so I just bought a yong heng

That is not the way it works. The faster the flow the better the cooling. That is a fact of physics. Faster water allows a much greater amounts of heat to be transferred because the difference in temperature between the heat source and the water is greater. It is hard to explain but because you can feel the heat in the slower water doesn’t mean more heat is being removed. It just means that slow water has more time to heat up and the more it heats up the less efficient the heat transfer.

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It isn't that simple. That might be accurate for the head end of the system (picking up heat where the delta is high) but I'm not even certain about that. On the OTHER end (shedding heat), more time may be needed to shed heat from the system as the delta will be much lower.

Strietwise is 100% correct. The faster the flow the better, ALWAYS. It is that simple.

If you need more time to shed heat, you have to get a bigger radiator/heat exchanger, add cooling surface or push more CFM air/heat exchanger cooling medium, but the answer is never, never, never, and I mean never slow coolant flow. A given radiator can only shed so much heat at a given CFM/ambient temp, the only answer is to either increase the radiator size or move more CFM, maybe both. The cooling loop will not dissapate more heat by slowing the coolant if your radiator is the bottleneck.

I see this all the time in PC liquid cooling, someone convinces themselves that somehow slowing the water down allows the system to pick up/shed more heat. It doesn't, it makes the system overall less efficient as you want to move cooler liquid to the head end and warm liquid to the heat exchanger/radiator as fast as possible. If you slow it down at the HX end, the temperature differential lessens as the warm liquid cools with less new warm liquid to replace it.

I ran into this problem with my Yong Heng radiator setup. The answer wasn't to use a slower pump, but to use high-CFM 110v fans to move more cooling medium (air) through the radiator. I'm contemplating getting a larger pump than the one that replaced the stock water pump or even adding a second radiator, but it seems to be working for now.

The ONLY thing that slowing down flow does is improve heat transferred per volume of liquid moved, but that's a useless metric to judge cooling loop performance.