The contractor is incompetent in the field of heat transfer.
The rate of heat transfer is increased by the velocity of the water in the pipe. I won't try to explain all of the formulas.
The rate of heat transfer is also greater when the water temperature is greater. Since higher flow will more quickly renew the hot water in the pipes, higher velocity will result in higher temperature in the pipe.
The higher velocity also increases the rate of heat transfer in the boiler, and therefore increases the efficiency of the boiler.
Higher flow and velocity are better as long as the velocity is not so high that it causes erosion in the pipes.