How are you calculating the RMS values?
If you're calculating the values from the raw sample data, but only displaying RMS values every 2 or 3 seconds, then you would be better off letting the driver reduce the amount of calculations that need to be done, by downsampling in 'Average' mode, and then calculating the RMS values from the downsampled values. You can still collect and store the raw sample values if they're required for later calculations. You can get both downsampled, and raw data from the driver, because the call back function will place the full sample value data in one buffer, and the downsampled data in another. You just need to setup the buffers and provide their location to the callback function.