It's amazing how much of end-to-end travel time is NOT the plane flying. It would be much more cost-effective to optimize everything besides the actual flight time for almost all flights. Things like:

* Airport-to-city travel time

* Airport security time

* Optimize boarding (very straightforward solutions but require passenger education)

* Speeding up taxiing / takeoff sequencing (not sure how but there's gotta be a better way than what we do now)

Each of those are challenging but would be more impactful (and more solvable?) than supersonic flight time!