Hi Ian,
Unfortunately there is no way to clear or reset the RMS register. This is a value that is calculated constantly and can't be reset so any change in the RMS will have this settling time with it. We specify 440ms in the data sheet but this can be less depending on how large the change is and where it happens in the waveform. The filters will always have some settling based on the waveform going in.
What is the application for this? Will it be used for a relay, breaker or something of that sort where you need to react quickly? What kind of changes in the RMS are you looking to track?
David