The problem is that it takes so long because they make a bunch of requests to the opt out endpoints of different services, which takes time. Of course, it is questionable why those requests are essential, but the point is that there is a reason to this madness.
But why do they block user interaction while doing that? You can call those in the background.
And why call an opt-out endpoint at all, after all there has to be a mechanism that prevents setting the cookie before the user sees the cookie banner. Just continue using that mechanism (e.g. gtag's consent default denied).
