You can now callset the ADC in freerun mode to get results as fast as they can be read.
Use adc_configure_trigger for this.
adc_configure_trigger(ADC, ADC_TRIG_SW, ADC_MR_FREERUN_ON);
To enable freerun on the ADC or to configure as needed. No need anymore to set registers.
There was a bug with this function before if called more than once, but it is now fixed. https://github.com/arduino/Arduino/issues/1819