tartare,
I have found myself staring at that question as well.
On what sectors does it become interesting to spend only three or four hours in the air, and sort out the 'timezone shift' on the ground?
Europe - Australia/NZ, possibly.
US West Coast - Asia and Australia, probably.
Europe - US East Coast (which would be about an hour's flight) ?
So much time is now spent on ground transport to and from the airport, "security" at the airport, taxying at the airport at 30mph, waiting in the take-off queue, then waiting in the landing queue and getting to the gate, disembarking, getting through immigration and getting one's luggage.....
Chopping a significant part off the 'in the air' time is only worth it over long distances.
And even then the time zone changes can negate much of the gain in travel time.
Look at Concorde. UK or France to the US you gained. In the other direction you still lost.