Another way to cut loading time in half is to eliminate carryon luggage. Have it checked at the gate.

Aircraft normally park nose-on to the terminal. Having a Skyway reaching the rear exit implies it extends the length of the aircraft and between the wing-tips of adjacent aircraft parking places. If you don't have skyways, just normal stairs, it is easy to have stairs front and back, which is why cheap airlines use stairs, to save time loading and unloading by using two doors.

Banning carry-on luggage might save minimal time loading (though I doubt it), but adds time (and cost) at checking in and retrieving luggage. Most regular travellers on short trips prefer to travel with hand luggage only.

Because of the shape of aircraft, it would be a significant engineering challenge to have two skyways. However many airlines do use two doors -- either two moveable stairs, or a skyway at the front and a stair at the rear.

In my experience, it doesn't make much difference to loading time. Loading time is dominated by a few selfish individuals who block the aisles whilst fiddling with the stowage of their carry-ons. This is not a problem with carry-ons per se (most people manage to stow their stuff while standing in front of an aisle seat so they don't block the aisle, and only take 2 or 3 seconds, so they also don't block their seating row for long) but with the way those people go about it. And it only takes two or three of them per flight to slow down loading to a crawl.

I'm not sure of the best way to fix it. I did once take a flight where the hostesses lined everyone up in boarding order (whilst waiting for a delayed plane!), so everyone could go straight to their seat without having to pass anyone else. Worked great for about the first (last actually) 13 rows, then some selfish idiot who had sneaked on ahead of his row stood in the aisle right near the front, carefully arranging bags in the locker...