Okay here's what I did to finally update Jellyfin to work with my 300,000+ size flac library.

I copied my 10.X whatever Jellyfin 3 times. So I basically setup 3 identical Jellyfin instances. I then upgraded each of them to the latest version. 2 of them upgraded but had horrible performance. The third one finally works the way it should. I have no idea why that worked but after seeing tons of comments that were basically "Idk mine just works" or "idk mine just sucks" it seemed worth doing.

So, maybe try that.

To be clear I didn't copy the libraries. Just the Jellyfin docker data.

Their new database is just bad.