Interesting info there from the Cabotage Kid, my own observation supports his argument.
Last year on a flight from Zurich to Bangkok in a SWISS MD11 the groundspeed was 1111 kph for over 2 hours after reaching cruising altitude out of Zurich.
This week, returning from Jo'burg to Kuala Lumpur in a Malaysian 747 the groundspeed was 1076 kph after reaching cruising altitude of 33000 feet, that lasted about 2 hours, from Maputo (Mozambique) to far side of Madagascar.
Later in the flight the groundspeed was 858 kph at 37000 feet, for the last 2 hours into KL, this was in a band approx 3 degree either side of the equator.
Please excuse the anorak response, I am a lifetime aviation nerd including 35 years as a ppl.