I’m using Quod LibetQuod Libet with a music library of 170 GB (~ 30000 tracks).
Its performance is outstanding. My PC is old and has weak hardware, but listing (or searching through) all those 30000 tracks only takes a few seconds (maybe 2-4 secs).
Unlike some, Quod Libet will scale to libraries with tens of thousands of songs.
This good performance was one of the main reasons why I chose QL, but I’m very happy with all of its other features, too. It’s one of those rare pieces of software which I consider perfect for me, and I can recommend it whole-heartedly.
I’m using it daily for many years, and I can’t remember any serious issues. I never experienced missing songs or freezes. A few times in all those years it crashed (mabye 10 times), but these were individual cases which I could never reproduce.
About your other requirements:
- it runs natively on Xubuntu (you can install it from Ubuntu’s packages)
- it’s gratis and Free/Open Source
- it uses GStreamer, so it can play MP3 (and all other file types you‘ve GStreamer plugins installed for)
- you can create/import/export playlists
- it’s not a KDE application (it’s based on GTK+)
So it seems to be a 100% match, according to your stated requirements.
It comes (nicely integrated) with the tag editor Ex Falso, which I’d also consider really good (I used it to organize all those 30000 tracks).
Screenshot, showing the album view (which is only one of several views):
(I recommended it also as iTunes alternative on Ubuntu.)