Now it is entirely my personal guess, but back in the day you actually had legroom and seats that reclined on planes. Now that extra room has been taken up by m

Now it is entirely my personal guess, but back in the day you actually had legroom and seats that reclined on planes. Now that extra room has been taken up by more seats. It is my belief that this condition, could explain a large portion of the time increase. As more seats leads to the combination of more people needing to board the same size aircraft, them needing to squeeze into smaller spaces (get up after sitting if they sat down before someone to their inside), and less room per person for carry on luggage so they sometimes need to find a overhead bin not directly above their row. As commentators have pointed out security at the gate probably has also increased time, I think number of seats per aircraft could also be a contributing factor.