Back on this topic... I have found a very nice solution (at least for me...) for this issue.
I think that the problem, on Volumio side, is to
restart Volumio each day (or each time you want to listen to music),
rebuid the full datase, even if nothing has changed; for me, this is a non sense.
This is what the “node” process does at each boot.
As far as I understand, mpd is very fast for updating the music library, but this is not the case for volumio, when this library is large. So, for me, the best solution, when using a NAS, is to have your DB located on this NAS, and not in Volumio…
For doing this, I use subsonic.
For me, this solution is perfect because my NAS (RPI4) already runs subsonic (http://www.subsonic.org/pages/index.jsp), which allows me to listen to my music from anywhere through a web interface. And there is a very nice plugin for olumio, named Volusonic, which allows volumio to stream music from subsonic. I gain also a couple of features from subsonic: random albums, newest albums, etc…
So, since I use Volusonic, I have absolutely no performance issues anymore! All you need to check whether you NAS can run subsonic or not.