Jim,
This is a cool find. The strange thing though is that not all installation have those QoS limits by default.
Below are the defaults for a fresh Untangle 15.1 installation (designated as "Home", in case that matters): Untangle 15.1 Default QoS Priorities.jpg
With those default even medium priority traffic can exhaust the bandwidth. But perhaps those defaults have changed recently and you are working off an older installation?