Here is what a Google search found.
"Copper/brass radiators are typically made with ⅜”, ½” or ⅝” cooling tubes. As you can see, aluminum radiators are made with much larger cooling tubes. These larger cooling tubes are the reason why aluminum cools better. And the reason aluminum radiators use larger tubes is because aluminum is stronger.Jul 8, 2016"
It's not that you want the radiator to slow the water down but the size of the cooling tubes should be just the right size that will allow sufficient time for the heat to be extracted from the water. Too fast equals poor cooling.I've never heard that you want a radiator to slow the water down.