I bet it's a minuscule amount. For a start, all they're shopping for is a shorter delay for the visa interview - maybe that is 1 month instead of 3 - and most people from countries that require visas will already be factoring that duration into their plans.

> how hard is it going to be for a motivated applicant to establish "proof of residence" in an arbitrary country

By residence they mean your legal status is a permanent resident, this is proven with a formal ID like green cards.