When I visit Special:GlobalPreferences, it takes several seconds until the "Select options below to be global" checkbox appears and the non-global options are deactivated. This should happen faster, ideally such fast that the user won't notice any change.
From T205991 I assume, that all tabs are infused at once, while IIRC Special:Preferences (which is much faster) infuses only the current tab. So this could be an option to try in making the initialization faster.