The passengers aren't all filing out the one door at the front in that situation. I'd say an average jet has at least 6 emergency exists (2 in the front, 2 in the exit row, and 2 in the rear) so nobody is more than a couple of rows from an exit in that situation, plus they are moving their ass and they are not dealing with luggage.
Which begs the question: why can't we just[0] replace the emergency exits with regular doors, have more jetbridges, and load planes faster?
[0] My use of the word "just" should betray the fact that this is far more complicated than I think it is. Obviously, this would be too expensive, impractical, and double the weight of the aircraft all to solve a very minor traveler issue.
Rear exits are already the same size as the front exits, but getting a jet bridge safely past the wing isn't easy so you'd have to mirror what European carriers do and use dual stairs.
If you're thinking of using two front exits, jet bridges also can't get too close to the engine so the only narrowbody that works for is the aging 757. Plus it doesn't really save time because the space between 1L and 2L is generally first class.
