You have the right forum as this problem does crop in here up all the time. I have just got back from an extended trip to Australia and observed the loading of: A388, A332, B738, E190 with both bridges and stairs at about eight or nine airports.
For the most part, aircraft are still loaded one person at a time. The 388s were loaded with three bridges: 1 for Suites (1st) on lower deck. 2 for Economy and PE on rest of lower deck. 3 for all upper deck. So for the greatest part of the aircraft, that's loading one person per deck at a time. However, the staff (SQ) did work hard to start boarding promptly and all four sectors left on time, even when at capacity.
The 332s had two bridges, one for Biz and then the rest. Some 738s were loaded with a bridge and stairs at the rear, some just with a single bridge.
All the problems of loading that I have seen throughout my travels were replicated. Which is to say that, even if the carrier tried to board by groups/rows/areas - as soon as the boarding process started most pax surged forward and jostled to get on. Only occasionally did I see the gate staff telling people to wait until their row was called. The discipline of queuing and waiting to board in the most efficient manner? Those days are gone.
Once on board, many were not aware of the protocol to get your bag into the locker and SIT DOWN! Or, stand into your row and wait for a gap in the line of pax going past you to other rows - and then put your bag up and SIT DOWN! Groups of tourists being herded by the guide (as they spoke no English) just did what the group did and then what they wanted and took up time. The CC knew that they could not enforce the concept of pax sitting in their allocated seat etc.
On one flight people had taken our row by mistake, it was lightly loaded and so the CC agreed that we should take their seats in the row behind.
So I suggest that even if the carrier had more bridges/stairs you could not speed up the people.