I was not only surprised how fast GIMP installed -- I was impressed it how fast it opened (you know, the first time) and it actually worked on my little Celeron.
(When this Mini PC was running Windows -- GIMP was out of the question.)
I'm running the latest Mate...
True. On my Win 7, it was like 2 minutes or more to start GIMP. This is amazing.
I can report the same issue with trying to install GIMP on a fresh, uptodate Ubuntu Mate via the Boutique software manager.
I simply used sudo apt install gimp and it worked immediately...
When I did a fresh install of Mate 20.04.1 and tried installing apps from the software boutique on first boot all failed but after a reboot they all installed without any problems.