This just isn't true. Sure, if you slow coolant flow more heat is pulled out. But doing that also means more heat is being transferred to the coolant in the block.
Rate of heat transfer is not linear. It's dependent in part by the difference in temperature between the two surfaces. Which mean as the coolant temp decreases in the radiator, the rate of heat transfer also decreases. Thus, the longer the residence time, the less efficient the system is. Increasing flow maximizes efficiency and allows the thermostat to do its job.