Non pilot speaking.
The last time I left JFK, the taxi time from push to turning onto the active was 55 minutes. All airlines do now build in 'delay' time to their schedule. on a short domestic route I used to take three times a year (LTN~IOM) the wheels-to-wheels time was usually 45 minutes and with usually short taxi's either end, so the sector was always marked as 60 minutes.
As to time in long haul, the supporters of Boeing used to make much that the 744 was a lot faster than the A343. On the LHR~JNB route which I know well, the difference allowed by VS, who operate both types on the route, is just on half an hour. That is: across some 5,000 route miles and 10.5 hrs, the Airbus would only take some 30 mins longer (to be fair, the company point out how much more economical it is on fuel).
Lastly, this change in schedules started because (1) the carriers wanted to minimise pax delays at the gate by bringing the scheduled departure time forward to get people to the airport earlier. (2) they lengthened the scheduled time when various govt and consumer organisations started to publish the on-time statistics. In the past decade, the UK train companies have all done the same thing!
So the room for making up time? They make sure that the time is already there before they rotate. I'm going LTN~MAD this evening and the advertised time is 2.5hrs. I expect we shall leave 'late' and arrive on time.