CaptainMiles

Yes, someone knows, just not someone here on FT. There are many factors that influence "most fuel efficient flying speed".

First you would have to define "most fuel efficient". Are you trying to maximize endurance (time airborne) or range (distance covered) or total operational cost ($ spent)? The speed that maximizes endurance is always lower than the speed that maximizes range, which is typically lower than the speed that minimizes cost. All of these speeds are influenced by air density (which in turn in influenced by altitude and temperature) and by aircraft weight. Further, best range speed will also be influenced by winds aloft. I don't think anyone here knows the altitude the aircraft was flying, nor the weight. MH may know the weight but I don't think they have shared that. And no one here knows winds aloft because those depend on altitude and location, which are all unknown. The pilots know all these factors, and can calculate and set the "most fuel efficient" speed.

Having said this, we can presume confidently that the flight had likely been planned to fly to PEK at a speed a bit above best range speed. Airlines typically do this. Best range speed minimizes fuel burn, but flying a bit faster, while slightly increasing fuel burn, decreases other costs such as crew pay, airframe hours, etc., and there is an optimal speed that minimizes the total cost that is slightly above best range speed. We can also assume that the aircraft had enough fuel to fly 6.75+ hours at this planned burn rate, and that the aircraft could cover the KUL-PEK distance and then some more.

Based on this, we know that the aircraft could fly a bit more than 6.75 hours (perhaps as much as 8 or 9 hours) if flown at best endurance speed. It would not get as far, but it would fly for a longer time. If the aircraft had more than 6.75 hours worth of fuel, than the best endurance time could be even longer. Exactly how much would have to be calculated by those that know more about fuel load and aircraft weight.