That super expensive KA dryer that was made by a commercial dryer manufacturer had separate motors for the drum and blower so it could provide reverse tumbling without sacrificing air flow. In dryers that don't have a second motor for the blower, reversing the direction greatly reduces the airflow because the blower is not spinning in the direction in which it was designed to turn for greatest air movement. When they pause to reverse, the heat source shuts down, then comes back on. With a gas burner, drying time is extended even more than with electric heating because of the burner shutting off and then relighting. With either gas or electric heating, the reverse tumble period with reduced airflow can mean longer drying times.