Could you elaborate a little on this, @h.g. ?
If people want to know more I would suggest quietly reading something like the B&K literature I referred to above or equivalents.
When you say the measurements take a long time to obtain are you referring to a stepped sine measurement? REW's default automated sine sweep takes only 6 seconds to complete and can be viewed without smoothing, and REW's RTA function can be displayed with 1/48th smoothing which shows almost as much detail at low frequencies as the unsmoothed sine sweep.
I wasn't referring to any particular type of measurement. To adequately resolve a low frequency high Q resonance you will need to use a large number of samples in the FFT. A single raw measurement will contain a significant amount of grass/wiggles/noise. Smoothing/averaging in frequency will reduce this but it will also smooth a high Q resonance into a lower Q one. If you ensemble average (average a sequence of independent measurements) the frequency resolution will be fully preserved and the grass/wiggles/noise will reduce but only slowly with increasing numbers of measurements. I don't know if REW can show the evolving averaged frequency spectra from playing something like white noise through the speakers but this would show the process nicely in real time as the wiggly spectra slowly becomes smoother as time evolves.
With regards to the number of measurements, is there not a risk that taking too many measurements spread out around the MLP will essentially mask the specific frequency and amplitude of the resonances heard at the MLP?
Google give MLP as My Little Pony. I am guessing LP is listening position but I am not what you consider might be changing over time.