I'd still question the usefulness of this rather arbitrarily number. The only reason I can think of where I'd be interested in how quickly people can navigate

I'd still question the usefulness of this rather arbitrarily number. The only reason I can think of where I'd be interested in how quickly people can navigate a site without having loaded all the content yet, is when optimizing a site for people with really slow/bad internet connections. But that seems like a whole other topic that has barely any influence on the goal of the comparison.