There are lots of fast, usable JVM gui applications.

Some OSS examples:

http://www.sweethome3d.com - This one has been the one app I've recommended to people outside the tech bubble! It also focuses on graphics and 3d.. really deserves to be showed off more often

http://xmind.net

https://dbeaver.com/

http://www.mucommander.com/

http://www.jedit.org

What they have in common is that they are old project - often more than 10 years old and are not on Github. Goes to show how important hype and trendiness is in our industry.

edit: fixed formatting

The JetBrains suite (ie IntelliJ, Datagrip etc) while not open source, are a testament to what can be done with JVM based GUI applications.

Have you ever used the merge tools? So dang cool.