Time sat spent in a metal tube isn't what most consider quality time, so I like to board as late as possible and sit in the boarding area with coffee and a paper. Now I find I have to get on the plane 30 minutes before push back or face being unloaded.

What specific reason forced the introduction of the requirement of having all pax on an international flight in/out of the US for 30 minutes before departure? As it's an FAA requirement I assume it has something to do with security.