Southwest has sort of figured this out. First of all, they use random boarding, which is almost the fastest, and the fastest of the viable options (good luck getting people to board alternate rows).
They also interleave slow and fast passengers. They let handicapped people board, then the first 1/3 of passengers, then families with kids, and then the rest of the passengers.
By the last 1/3 of the passengers, it's mostly center seats, which means high parallelism for those passengers.
"good luck getting people to board alternate rows"
Do you know right now if boarding group D is people from the back of the plane, people with window seats or odd rows?
Not sure which method each airline uses but the problem is that all of them let their premier members board first regardless of seat, and they let groups board together regardless of boarding group, so it screws the whole thing up.
They also interleave slow and fast passengers. They let handicapped people board, then the first 1/3 of passengers, then families with kids, and then the rest of the passengers.
By the last 1/3 of the passengers, it's mostly center seats, which means high parallelism for those passengers.