The faster flow rate will heat more quickly, but not by the factor of the flow rate. It's simply more efficient to transfer heat by a small amount that keeps the temperature differences small. A slow flow will be less efficient because the water gets heated hotter and therefore accepts getting more heat a little less (due to a smaller temperature difference between the flame/pipe and the water) and also loses more heat down the pipe (due to the greater temperature difference between the water and the pipe/ground).

However, I don't know how big a difference this is and you are welcome to experiment with both flow rates and let us know what you find out!

Richard