A service where your limo drives out to the aircraft, with all searches and paperwork pre-done, would have about the same time gain as going Mach 1.7 vs. Mach 0.85.
And probably fewer regulatory hurdles (and I don't mean that because they wouldn't be considerable...)
Underrated observation. The low-hanging fruit is all in the office/home-to-takeoff and touchdown-to-office/home blocks on each end, not the time in the air. The commute, checkin, security, airport transit, boarding, and taxiing are the time-sinks worth optimizing.