Many know that Southwest Airlines has some of the best on-time records and plane turnaround times. Some of this comes from the fact that without reserved seating, people can board the planes more quickly.
It seems to me it should be possible to board planes quickly even with reserved seating. Here's how...
For a simple system, draw on the carpet a diagram of the largest plane that uses the gate. Except put the rear of the plane up by the door, with numbers counting down to the front. Have a 2nd area for 1st class if you need to keep them boardig at their convenience. This can be just a line with row numbers, and a marker that puts window seats near the line, aisles further away.
When boarding is called, passengers stand in the line for their row, and sorted internally as noted, so window seats go first. Then just empty the line into the plane. Being out of place in the line will be very obvious on the plane. If people line up over their row number, you'll never wait while people load their stuff to get to your row, unless you're late for the boarding call -- which few people are today due to crazy security rules.
You hae to decide if premium frequent flyers and "people needing extra time" should go first or not as they do now. My view is that the pre-boarding needs are minimal, and that even a slow child or senior will be better placed with their row than pre-boarding, but pre-board can still be allowed. I also think the frequent flyers would rather have a plane that boards and leaves quickly than get on first, except for one issue -- overhead storage. However, even if you let us get on first, doing this for the rest of the passengers still will streamline things.
You could also just print a series of numbers on the carpet. People would be given a card with their number and expected to stand by it, showing the card. The cards can have a clear colour code making it impossible to hide which group of 12 you are in. In this case, you can assign low numbers to 1st class, pre-boarders and frequent flyers, and just sequence up by seat otherwise. Again, zip on the plane almost as fast as leaving it. That means boarding closer to take-off, and faster turnaround, which is good for everybody.