Tankless Hot Water Filling Tub - GPM?
Results vary. A tankless heater I've had for many years regulates the output flow to assure that the output temperature is as hot as expected, however, not all tankless heaters do this.

The "installation instructions &amp; homeowners manual" document from Rheem for the RTEX-11 states the following (page 7):


  To ensure the optimal temperature output and overall performance of your tankless water heater you may
  require a flow regulator. These flow regulators are installed on the outlet connection of your tankless water
  heater and limit the maximum volume coming out of your unit to a specified flow rate to prevent the exit
  temperature from becoming too cool. To learn more about flow regulators or find out how to purchase one for
  your tankless water heater contact Manufacturer’s national service department at 1-(800)-374-8806.


Based on that statement I expect that you could manually reduce the flow by partially closing the hot valve on your tub filler until the output rises to a temperature you like. Or, if you don't want to worry about that, install a flow regulator device to force the issue.