Also for some longhauls you have curfew in both ends hitting you bad due to the time-differences. For example you can not have any flights leaving JFK for LHR between aprox 11:00 and 18:00 due to arriving at LHR nighttime. So if the morning flight JFK-LHR got an expected delay of 3-4 hrs, it would probably be 10-11 hrs delay in the end.