Q: When iTunes Match is on, iTunes becomes unusably slow

I have a little under 15,000 tracks (music, a few videos) in iTunes Match. I'm using iTunes version 12.3.1.23 in El Capitan. The problem is that, when iTunes Match is turned on, iTunes becomes so slow that it's impossible to use. Every time I do anything in iTunes, I have to wait for the spinning wheel to appear, while iTunes hangs for 15-30 seconds. This happens with everything, from selecting a track, to scrolling down a playlist, to even just moving the pointer around! Yes, even moving the pointer causes the spinning wheel to come up for a little while.

I can still play music, but I can't navigate around the app.

As soon as I sign out of my iCloud account (thus turning off iTunes Match), the problem goes away.

Notes:

• It's not doing any massive downloads. I have it set to keep all my tracks in the cloud.
• I've tried removing all iTunes prefs, cache, etc., signing out of iCloud, then rebooting my computer and going back in from scratch. As soon as I sign into iCloud in iTunes, the performance goes back to bad again.
• This is happening on my iMac and my MacBook Pro, on my home and work networks, so it's not a network or a computer-specific issue.

Is iTunes Match just having major performance issues recently in the current version of Apple's software, or could there be something in my library that is causing big problems?

Thanks for any help!