I don't know if anyone goes to Flightaware on any given day to look up random flights (of course, I do, because I'm a geek and/or I get bored). But if you look at AAs flights, between scheduled departure time and actual takeoff time, it's typically 15-20 minute gap, maybe 20-40 at JFK. Looking at UA's, DL's-not so much. Maybe 5-7 minutes?

I had always believed that for whatever reason they decide to go slower out to the runway. That's nice and everything, but it doesn't help your on time record.