The time taken to board and disembark does go up the longer the fuselage and passenger capacity is, such as the longer 737-800, in fact for all sorts of subsidiary reasons it almost seems to go up by the square of the fuselage length, hence why those airlines with the 757-300 found that because of its extended boarding times etc it was really a stretch too far. The 737-900 seems to have had the same reception from operators, whereas the wider A321 does not seem to have suffered from this to the same extent.
In my experience it takes a hell of a lot longer to get on and get off an EI A320 (never mind the 321) than it does an FR 738.