A plane on the ground is a plane not making money for the airline. That is the reason why airlines try to turnaround their planes as fast as possible.
Disembarking is simple and happens efficiently: passengers milling out of the plane starting from the front to the back, aisle to window. It is boarding where all the confusion lies. There are three predominant patterns used by most of the airlines: free seating, back to front and zone based seating. United is the only airline that uses outside-in seating.
Southwest uses free seating. The main problem with this approach is that everyone wants to sit in the front of the plane (less engine noise, faster to disembark, less lateral movement, etc.). Also, no one wants to sit in the middle seat. This means people start filling the window and aisle seats from front to back. The problems with this approach are that people have to wait in the beginning since others are holding up the line as they find their seats up front. Later on, the process speeds up as people start moving backwards. And, at the end, it slows down again when the middle seats get occupied. One advantage is that there is never a situation where two people have to get up to let a passenger get to his/her window seat.
Most of the other airlines seat from back to front in zones. This causes packs of delays within the zones as people board in random order within the zones. Add to this mix, the elite travelers who board prior to all others. A lot of elite travelers prefer the aisle seat leading to more delays as the other seats are occupied.
United's approach loads the windows first, followed by middle and then the aisles. This would mean that there is no need for any passenger to get up once seated. But, unfortunately, the elites mess this up too. The elites board first and could be seated anywhere on the plane.
We could come up with all kinds of sequences to make boarding as efficient as possible, but, still we will have issues with each of these approaches. The main culprit being carry-on luggage. Finding space for carry-on luggage is the biggest concern for all passengers, and hence the rush to board.
In an Utopian world where passengers did not carry any luggage on them, boarding would be a cinch. Any of the above approaches would work pretty well.
One obvious answer is to eliminate carry-on luggage, especially the ones that fit in overhead bins. Laptop bags, backpacks and purses that can fit under the seat and could be fine. The big increase in carry-on luggage has happened since airlines started charging for checked-in baggage. It would be interesting to see how this has increased the boarding time. If it is significant, then, airlines could charge for carry-on luggage too. This would bring us back to the good old days of no charge luggage. Airlines can make more money from baggage fees, as well as increase the turnaround time.
Another answer is to open all doors and load passengers from all of them. Especially on the wide body jets with two aisles, opening doors on both sides will speed boarding incredibly. For the smaller planes, opening doors in the front as well as the back would do the trick. But, that is not really possible since all airports have only one jet bridge assigned to a gate. A redesign of the gates and bridges would be very expensive.
A few airlines are still experimenting with various strategies for boarding (alternate aisle and window, back to front, eliminate elite early boarding, etc.). But, the airlines are knocking on the wrong door. The boarding process is very well optimized. It is the carry-on baggage issue that needs to be addressed by either eliminating overhead bins, or by reducing the carry-on luggage. This would either mean charging for carry-on baggage or eliminating them completely.
It would be interesting to see if there has been an uptick in boarding times since baggage fees were instituted. If there is, it makes sense to address this problem by focusing on the carry-on luggage.