@Carmelo_DrRaw I tested:
https://github.com/Beep6581/RawTherapee/releases/download/nightly/RawTherapee-dev-5.4-271-gfde0dccd.AppImage
Clicking on a main tab now takes 1 second. That is still slower than a native build, but a great improvement 
What was the cause of the slowness and how did you fix it?
The console still shows:
Gtk-Message: 13:26:32.026: Failed to load module "canberra-gtk-module"
which probably means that audio feedback won’t work.