> the Linux version starts up faster than the Windows one.
Indeed. 4-5 secs to open GIMP on Linux here on HDD, and practically instant once cached. Maybe try and start with some command-line options. Useful ones in your case:
-d, —no-data
Do not load patterns, gradients, palettes, or brushes. Often useful in non-interactive situations where startup time is to be minimized.
-f, —no-fonts
Do not load any fonts. No text functionality will be available if this option is used.
-s, —no-splash
Do not show the splash screen.