Music does not play for large collections

This is *not* a duplicate of #1531650.

Untar the attached 7000 "song" collection and wait for mediascanner to settle down, then start the music app. Once the song list appears, scroll down a page or two and tap a song. After a delay of *several seconds*, the player finally appears, but is unresponsive. The song isn't played, and the play button does nothing.

Go back to the song list. In a terminal, cd ~/Music/music-no-artwork, and remove all the single letter directories except for 0. (Or move the directories to somewhere where the mediscanner won't index them.

It takes ages (10s of seconds) for song list to refresh and, during that time, the app is catatonic.