I think AA is doing something smart here. If someone is fumbling with a carry on trying to get it to fit in a small place etc., they hold up 20 passengers. The problem is that the next person that will fumble with their carry on is still on the gangway. They board and hold up 20 more passengers...and so on, and so on.

Makes a lot of sense that all of the "fumblers" would be fumbling at the same time. The pure physics of the situation says it could not really lengthen the boarding process...but could at times shorten it big time. Also, seems like over-sized carry ons and so forth would be easier to detect if they were all being boarded at once.