Thank you for trying to help me with that.
Actualy, it's very strange, but since I faced this issue, the scanning speed has improved. MusicBee doesn't rescan every file, only the new files, which obsiously takes more time.
I also tried to use an ethernet RJ45 cable connected to the box instead of scanning by WIFI and it's faster, of course. I guess I should scan the whole collection this way and then use WIFI only when I will add just a few albums.
I think that jRiver is faster because it's not connected the same way to the NAS. While MusicBee considers the NAS mereley as a remote hard disk and has to scan every file in it, which takes time, jRiver can connect to the media server generated by the NAS itself : I guess it means part of the job has already been done by the server software (Minimserver, Plex, Emby, etc.) directly on the NAS. That's why it much faster.
Maybe MusicBee could be improved by adding a media server feature. Yet, I guess it must be quite something to set up and that Steven, unfortunately, won't have the time to do it...