I recently flew CX SYD-HKG and the plane arrived 45min early - it arrived at 4.15am instead of 5.00am. I wondered why the plane didn't just fly a bit slower and land on time? There would be no problems with landing slots at that time and it would have saved fuel. Why would they not keep to the scheduled time - even with tail winds, can't they still just fly a little slower?
Aircraft drag curves look like an inverted bell. Normal cruise speed is slightly faster than the min drag point, but only slightly. In cruise it's quite normal for there to be as little as a few knots between cruise and min drag.
At cruise speed, if you have speed excursion onto the high side, you get more drag, and the aircraft will slow. Conversely, a small speed reduction will give less drag, and the aircraft will tend to go back to the original speed. That's without any thrust changes...the aircraft is speed stable.
Back at min drag, and those same speed excursions have a different effect. If displaced faster, you get more drag, and the aircraft will tend to decelerate. But, if displaced to the slow side, you'll also get more drag, and the excursion will increase.
Slower than min drag, and an increase in speed will give less drag, and so the speed will increase further, whilst on the slow side the opposite happens, so it's speed unstable in both directions.
If you get more than a few knots below min drag, you may not have enough power available to correct any speed excursion.
The upshot of all of that is that there is only a very narrow speed range that can reasonably be used. To correct the timing error you mention would need around .09 mach slower than usual...most likely only about .03 or .04 would be available. If you must fly slower, the only way to do so is to fly lower...and that plays havoc with the fuel burn.
Additionally, around the min drag speed, the auto throttle tends to become more active..the more active the less efficient.
Flying time also costs money. We use a number called a "cost index", which balances the fuel vs the flying time costs. A cost index of zero will give the least cost...but also normally only a .02 mach speed reduction.