This is sort of “hurry up and wait”; every time I feel like I got through the gate quickly, I just end up in line on the jet bridge or waiting 2 minutes for someone to fish every variety of item out of their overhead bin while the whole plane waits for them.

The cause of slower boarding is BAGGAGE FEES. Airlines are penny-pinching, people bring everything on board to compensate, and struggling with luggage is slow as hell.