Airstairs are typically narrow. I can overtake somebody on a wide staircase when neither I nor they are encumbered with luggage, but it's really not safe to try overtaking somebody descending airstairs when I or they are carrying bulky luggage. Thus, if somebody is walking down airstairs slowly (e.g. a small child that finds the steps a bit steep or somebody struggling to carry large hand luggage), everyone behind *MUST* wait for them to reach the bottom of the airstairs. However those people who descend stairs slowly can often move along a flat and level surface without a steep drop (e.g. aircraft cabin aisle or airport apron) much more quickly.

The result, is that when exiting an aircraft, you have a pathway with different speeds - a 1st section where many people can move fast (cabin aisle) followed by 2nd section where people move slowly (descending airstairs) followed by a 3rd section where people can move fast (airport apron) - i.e. the pathway goes fast - slow - fast; this is different to boarding where the pathway is one of fast - slow - very slow.

The conclusion is to increase through-flow solely in the 2nd section by increasing its capacity - namely use doors on both sides of the cabin to allow pax to descend, and loosen the bottleneck around descending airstairs

A similiar solution exists for the toll booth section of a motorway - the road becomes much wider at the short section where drivers have to slow down, but narrows again shortly after everyone clears the toll booths