Thanks for your reply - I realise that 'perceived' time is greater at higher altitude, but is there a real difference in journey time - ie is it further at higher altitude, or is this just an impression?
I also accept that the potential speed of a Tornado is in excess of the speed of sound, but normal operations are at subsonic speeds (there was no sonic boom), but is there a limit (either physical or legislative) of actual speed between 500 ft and 50,000 ft?