This concept, known as turnaround time, has been studied by airlines for a long time (it was even presented as a study in my MBA program) because they honestly do want to have the planes on the ground for the least amount of time possible. They're only making money when they fly. That being said - the multiple jetway approach certainly helps - especially with the A380 flying in Prestige Class on KAL and boarding directly on the second level.

As much bad press as Boeing is getting recently, I think boarding the 757's in the front middle (in the middle of C+, where there are three rows of C+ and 5 rows of F to the left) cuts out a significant amount of boarding time because you don't have to wait on F to get situated to continue boarding. I don't believe the A319's, 20's and 21's would even have that as an option.