Merry Christmas, Rob!
Periodically throughout the 1970s Cunard did explore slowing her down and actually introduced a six-day crossing schedule for 1972 but that lasted for the year because it deterred businessmen travelling. This was a market Cunard was keen to exploit as a relaxing alternative to flying but that needed to promise the businessman only a minimum working days would be lost - promotions such as "the long weekend", "fly one way free" and "take your wife free" were all designed to promote QE2 as a true alternative but they needed a five day crossing.
The departure of France in 1974 also had a bearing of QE2 maintaining the fast Atlantic crossing in order to attract France's clientele.
And while fuel may have been saved their was strong objection from the crew for a longer crossing (highly unionised so best not to rock the boat) and, while a longer crossing would allow Cunard to charge higher fares, the market throughout the 1970s would not tolerate higher fares which would only deter people so I guess Cunard thought it was easier to keep throwing her across the Atlantic at high speeds as their would be too much of a knock-on effect if they extended the crossing length.
She was eventually slowed down to six-days in 1997.
Cunard today are missing a trick - sending QM2 across in five days as a one-off would make it the fastest Atlantic crossing of the 21st Century. I'm no marketeer but surely they'd fill that at a premium which would more than cover the extra fuel cost. Question is - is she capable of five days crossings now?
Michael