Critical mach
The trident had a droop limitation of fl230 iirc as the flow would become locally supersonic, ignoring it led an involuntary descent through the Clacton hold before the crew could recover the aircraft.
There were a couple of captains that would descend a few hundred feet at cruise level to accelerate the aircraft and allow it to fly with a lower nose attitude would reduce parasite drag and allow a higher cruise or lower fuel consumption.That's the theory they spouted..there is also the ram air effect to take into consideration.