De facto:
I save time on the ground and try to get shorter routings and optimum levels to save time...not flying at m.080/320.
Seems smart to me.
Saving 1 minute on a flight but monitoring the speed like crazy when flying at mach 0.80, no thanks. it's not worse it.
Yes there is a cost index, but .76 and 280 are speeds that respect safety (turbulence), and like to use them.
However at high speed, the cockpit noise difference is more important than in the cabin... Smooth transition is the key, hard to beleive the speed is a confort factor when no turbulence...