I may have missed it (this is a long thread) but no one has mentioned that the rate of heat transfer increases as the temperature differential between the hot object (engine in this case) and the cooling fluid increases. So that would support the postulation of not moving fluid slowly through the block if you want efficient heat removal. You wouldn’t run a small radiator fan(s) so that the air has more time to remove heat from the radiator because we all should know that would result in poor heat extraction and thus an overheated engine, so why would you reduce the flow of the water pump?