I finished installing Catalina around 1 a.m. then went to bed. When I got up, my computer had alerts about needing to shut down all apps because my iMac (with 64 GB of RAM and 900 GB of available storage) had run out of system memory.
All my apps were still up, although the Music app was had quit and lost all its configuration. The ‘runtime’ command showed that my computer had not rebooted since the install, so I figured Music must have crashed, and that everything had somehow returned to normal. I let Music run for a while, reconfigured/resized it, allowed album art to re-download, then relaunched it.
I returned home this evening and my computer was using up all 64 GB of RAM with 70 GB of swapfile usage, all of it going to the Music app. The app was also constantly using up 150% CPU and causing stutters throughout the system. Every once in a while, album art was flickering in and out of my playlists, and the activity window was showing “Loading artwork…” over and over.
Shutting down the Music app instantly released 120+ GB of memory. But then all the artwork is gone once again.
There is something seriously wrong with the way the Music app is dealing with album artwork. It might be a conflict with artwork downloaded in iTunes on earlier versions of macOS...