I would agree, in theory, but know that UA used to focus on efficient turn times for aircraft. Padding can be beneficial for the first flight of the day, or where an aircraft sits for a while. But just a few years ago, TED planes had a goal of a 36 minute turn time, and I think that was true for the other Airbuses. 757 aircraft had a 45 minute turn time. The least profitable time for an aircraft they found was when it was sitting at a gate and not transporting. They even paid Mckensie Consulting a bunch of money to help them determine this. And then training programs that mirrored NASCAR were implemented.

So knowing that deboarding takes a good chunk of time out of that, its hard to stretch out boarding times too much, otherwise it will be a waste.