Coolant flow

I would think that the rad would have to have the capability of cooling faster than the heating rate. If the heat is building faster than it can be dissipated through the rad, it's going to overheat. Speeding up the flow, if the rad can't keep up, eventually it's going to overheat. Slowing down the flow, if the engine is heating faster than dissipation, it's going to overheat. One has to look at the complete design to determine the effectiveness of the cooling system.......Not just one aspect of it. JMO :)