itunes song play delay

Extremely vexing problem with delays between when I double-click a song and when it plays, during which time I get the spinning beach ball for anywhere between 5-30 seconds. I've had the issue since getting this MacBook last year (onto which I restored an existing iTunes database from my old MacBook), and it has seemed to steadily worsen, with delays now regularly of 15 seconds or more. I teach dance classes using iTunes playlists, and it's gotten to the point of being embarrassing, with students looking at me expectantly during these interminable silences.

Doesn't happen with every song, nor the same song every time. Even after a year of daily iTunes use I'm unable to recognize any discernible pattern. I've reported to Apple, and we've attempted various troubleshooting methods, but thus far they've been unable to help.

I've just over 15K songs in hundreds of specific playlists, sorted by dance style, tempo, etc., so to re-create from scratch would require more time than I've got.

Any assistance, insights, good jokes for the silent moments, would be greatly appreciated.

Specs:

iTunes 12.7.3.46 running on

MacBook Pro (15-inch, 2016) / macOS High Sierra 10.13.3

2.7 GHz Intel Core i7 / 16 GB ram / 500 GB flash storage about half used