Hey Dyslexicant,
I have no stake in any one particular cause, like I said, I had to delete all of mine and reload and the application works much better. So since you do not have playlists, did you do this? Move your music to a backup, delete iTunes, reload iTunes and then add all your music again? It definitely has something to do with upgrade to iTunes 12, so I would assume it has something to do with the way Apple Music was built in to the architecture of iTunes.
But when I did do all of the above, it worked normally again. I've been adding playlists and there definitely is starting to be some minute delays, for example, in switching from track to track in the editing process, so there does seem to be some correlation. But I have no idea what the heck is the problem.
I have noticed that when I use the Column Browse function to access and play albums and songs, it is different from having playlists of albums in that if you are playing a song and start browsing around to different albums and music, the album will stop playing on completion of the song or jump to the new album you are playing, its very irritating.
Anyway, just saying, obviously that has nothing to do with your point. But how many songs do you have in your library? How do you access your music? Through albums?