I have to disagree with Jean-Lill. In my experience, unallocated seating is slower, especially with a jet bridge, because the first people to board sit in the first rows, so everyone has to wait for them, then the next lot sit in rows 3 and 4, so everyone is again waiting for them. Then the next lot sit in row 5 and 6, again a blocked aisle, etc etc.
And then some people will be saving seats for the rest of their group, but when they arrive they decide they want a different seat with a better view (not over the wings), so they try and move, creating more blockages in the aisle.
Then as you're finishing, you have a family with two small children who want to sit together, but you can't find seats together, so you need to ask people to move slowing things down again.
Then, if there aren't many people, you start having to ask people to move to balance the weight.
All this would be faster if it was done at checkin and allocated seating.
The claimed benefit that people are at the gate early only works if EVERYONE is at the gate, because on most occasions after the boarding is finished, we're still waiting for missing people who have checked in (even with luggage), but who are not yet on the plane. We can't close the gate until 10 mins before departure time, so everyone's on board waiting. Then, when you can wait no longer, you still have to find their luggage in the hold before leaving.
Two doors are much faster, yes. People who board at the back will usually walk towards the front which helps a lot making boarding time perhaps even more than twice as fast.