On a lot of plane layouts, the unwashed hordes enter between coach and business/first. So no mingling.
That’s exactly the policy of Ryanair. They very openly state that the free-for-all is quickest. Their turnaround of aircraft is 25 minutes, no matter what airport. If the airport can’t assure them that baggage and fuel won’t be done in that time, they just don’t fly there. And in my experience, they’re right - the doors are almost always shut before well before the baggage is all loaded. (And the carry-on is as heavily restricted as any airline, preventing first-class hogs putting all their worldly possessions into the first two-thirds of the lockers.)