Why do FLOWKOOLER hi flow water pumps cool BETTER THAN OEM?

Whether it is in stop and go traffic, a slow moving parade, in line at a car show, off road on the trails or hauling a heavy load up a grade all cars face diminished airflow through the radiator at slower speeds.  FlowKooler hi flow water pumps push twice the coolant through the system at these lower speeds and help the engine transfer the heat.  The result is engine temperature reductions of up to thirty degrees, increase system pressure in the engine block which reduces ping and improved flow efficiency which means more horsepower. 

Doesn't the coolant have to have more time in the radiator to cool?

In a closed loop cooling system sending coolant through the system fast or slow does not change the thermal dynamic properties of the metal in the radiator.  Simply put, you have a better chance of keeping your cool with the coolant in the area of the car where the heat exchange is really happening!