Actually, WN pilots have a pretty well-known reputation for taxiing at higher speeds than other airline pilots. It's somewhat ironic that you use WN as an example as their on-time ratings have been less than stellar in the last few years. Airlines can't control things like congestion and ATC issues. I don't see an issue with airlines using realistic times which can vary with the time of day based on past experience. The fact that they use different times depending on departure time is a pretty good indication there's some actual science involved here. Again, if the goal is just to simply pad, why wouldn't DL just slap a 2 hour time on all the flights and be done with it? Also, airlines have a built-in incentive not to pad schedules too heavily as it decreases aircraft utilization which is ultimately going to impact the bottomline.