* Not all of first class boards early, many flyers stay in the lounge till later and then hold up the aisle getting their bag into the overhead
* Passengers board together regardless of boarding order. Friends flying with an elite member can board early and not with their group.
* When the overhead is full passengers must swim upstream to bring their bag to be checked.
* Although there are plenty of bathrooms near the gate, many passengers insist on using the lav the minute they board. They again end up swimming upstream holding up the queue
* Seat stealing - whether intentional or not, some passengers help themselves to a roomier exit row seat (or on one occasion I've seen them in first class) hoping no one shows up to take that seat. Once the passenger with that seat arrives, the original passenger has to get their belongings, perhaps clear the overhead and again possibly fight their way against people boarding.
Southwest's self serve seating helps in some regards, but this style of boarding will never be used my the major carriers - at least in the U.S.
On a recent flight using Azul (Brazil's "Jet Blue", same owner), people were boarded via the front and back at the same time. First time I had seen that, though not sure if it was any quicker. Perceptions of quickness are often dependent on where you are sitting.