I think what you discovered is that GIMP is written primarily for Linux, rather than that Windows is shit. I mean Windows is shit, but that's coincidental. GIMP for Windows probably loads slowly because it "has to" load GTK and all its components like Cairo, but on Ubuntu these things were loaded already when the desktop started up.
I put "has to" in quotes, because it could have been rewritten to use Windows APIs, but that would be, you know, a rewrite. And GTK was made for GIMP. So like a lot of ported Linux programs it's basically going to bring half the Linux desktop with it so that the devs don't have to leave their comfort zone, and as a result it loads slowly. (And then the next one, Inkscape maybe, will also load slowly, since it loads its own versions of the same things.)
In my experience a lot of the difference in performance of Windows vs Linux in programs which are designed for Linux is in the file system. Linux is really good at handling a bazillion tiny files. Windows is really not. I'm sure that in bulk both systems are roughly as fast, but when loads of small files get involved Linux absolutely smokes windows.
I think what you discovered is that GIMP is written primarily for Linux, rather than that Windows is shit. I mean Windows is shit, but that's coincidental. GIMP for Windows probably loads slowly because it "has to" load GTK and all its components like Cairo, but on Ubuntu these things were loaded already when the desktop started up.
I put "has to" in quotes, because it could have been rewritten to use Windows APIs, but that would be, you know, a rewrite. And GTK was made for GIMP. So like a lot of ported Linux programs it's basically going to bring half the Linux desktop with it so that the devs don't have to leave their comfort zone, and as a result it loads slowly. (And then the next one, Inkscape maybe, will also load slowly, since it loads its own versions of the same things.)