There is an option when saving: Save this XCF file with better but slower compression. I have found that this does not slow the process down enough to bother me, and am wondering if there is a way to set GIMP so that this always takes place, IOW, as default?
Set better compression as default in the Save dialogue?
Nothing that I know of. I had a good look around and no obvious settings.
You could put that in as a request at
https://gitlab.gnome.org/GNOME/gimp/issues/
I think it should be default. To save as a legacy Gimp 2.8 .xcf you already have to make sure all layers, modes etc. are compatible, why not compression as well.