It's by aircraft type. UA has it's standard 757 and the PS 757. UA's standard has 182 seats. CO has the 757-300 with 216 seats and only 1 aisle to move thru. It takes way longer to board a single aisle aircrft with 216 seats than a dual aisle aircraft with 174seats (sme 767) even if the dual aisled, smaller capacity aircraft is a webody.

So the 757, in any configuration except PS takes longer to board than any 767, and in practive, longer than any other aircraft in the fleet due to the massive quantity of people with the single aisle that creates bottlenecks. The unction of boarding tim is linked only to aircrat model..

Remember, these times are for full turns. If the plane arrives late, boarding time is 1/2 of the equipment service time. Ever board a UA 757 when the arival was late? It is chaos and usually doesn't make up any of the lost time on the ground.