LibreOffice definitely feels sluggish.

I give it a pass because it's doing complicated things, it's free software and I also temper my expectations slightly because it's a Java GUI application (and I expect those to be slow).

I would certainly not expect it to be more performant than Office '97.

LibreOffice is derived from OpenOffice.org which is derived from StarOffice which predates Java. When it was acquired by Sun and open-sourced, they added some optional components implemented in Java, but the core application is not a Java application. The GUI is not Swing but their own custom GUI framework (not based on Java).

Libre/OpenOffice is a C++ application which uses an homegrown cross-platform GUI toolkit. There were only some minor components written in Java (like mail merge), and I believe LibreOffice has replaced some of them with native code.

I can't find references now, but Gnome or Ubuntu had a phase where they were booting the desktop with under 128MB of RAM.

It would be great if LibreOffice had a spurt of speed-ups. Kind what the browser wars also competed fiercely about.

LibreOffice may be slow, but is UI is not build in Java, 90+% of LibreOffice is built on C++.

Being written in Java there is no surprise here.

Java is optional, not used for any of the core office functionality. You can build and use LibreOffice completely without Java.

If you want LibreOffice with less Java, you should try Collabora Office. https://www.collaboraonline.com/blog/collabora-online-now-av... (I'm not affiliated)