I’m using [**Quod Libet**](https://github.com/quodlibet/quodlibet) 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](http://packages.ubuntu.com/trusty/quodlibet))
* 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](https://quodlibet.readthedocs.org/en/latest/screenshots.html), showing the album view (which is only one of several views):

> ![Quod Libet: album view][1]


  [1]: https://i.sstatic.net/Skf7w.png

---

(I recommended it also as [iTunes alternative on Ubuntu](https://softwarerecs.stackexchange.com/a/180/60).)