Scanning a Premium Music library can take a very long time.
It may take forever if there are folders full of tracks from mixed sources (i.e. not from a single, known album) and more than ~65 tracks in a single folder.
A strict folder organization by Artist > Albums > Tracks is highly recommended
https://support.plex.tv/articles/200265296-adding-music-media-from-folders/
If your files are not very well structured, a non-premium (last.fm) library is probably better to use.
