Bruce, was there a pressure reducing valve on the home.
I had a similar issue and come to find out the reducing valve was cranked down to low, they need a certain amount of water pressure and volume to function correctly, but I don’t remember exactly what that was.
Don’t know about the Rheem, but have had experience with Polaris. They do lower the pressure in order to slow water flow to heat it. While the heating is fast, it is not instant. The ones I have experienced, were a number of years old so new technology may have changed this.