I think that when Dulles used mobile lounges the published departure time was 15 minutes before the planned push back and the published arrival 15 mins after scheduled chocks on (planes appeared to achieve 10 minute turnarounds). So longer journey times.
Mind you there is a time penalty for coached flights. (I flew KLM Cityhopper back from Schiphol yesterday with coaches at both ends. It was fine at AMS although on another flight a connecting pax arriving more than 15 minutes and a tight transfer passenger missed their flight. She would have made a flight from a gate. At LHR T4 it took me 30 minutes to reach passport control including a long walk, and I was the first off the second coach. Rant over.)
The idea was to run the mobile lounges from a centralised holding area so eliminate long walks to distant gates. I believe that they are still used for arriving pax requiring federal clearance at Dulles. I think that in the 80s Pan Am and Port Authority used mobile lounges for ferrying to planes parked at remote gates, believing them to be more comfortable than buses.