I have heard that a thermostat will slow the flow, but that does not matter. If the water is going through the radiator "fast", it is also going through the engine "fast", so nothing is gained or lost by slowing down the water flow rate.
Think about it like this: If the engine runs for an hour, then water has been picking up heat from the engine for an hour, and giving off heat in the radiator for an hour. All the water is moving the same speed, so makes no difference.
Dennis M. in W. Tenn.