A 1971 BOAC timetable I have here gives the departure time of BA591 to New York and on to Australia as 1100 until 24 April, then 1200. Although this was the time change date it was not the only reason, as the flight was now 2 hours later at New York and beyond.
It was very closely shadowed on certain days by Qantas 531, a 707 which left Heathrow at 1145, and made all the same stops apart from serving San Francisco instead of LAX. At just about all the stops the two aircraft were on the ground together, the Qantas was 15-20 minutes ahead of the BOAC summer times throughout. I wonder if they used to keep in radio contact along the way. Also makes you realise the performance of the two types was identical, certainly as far as scheduling was concerned.