I hate to say it depends, but it depends. Are you using DI water? If you aren't, you should. Are they detail conscious, meaning not worried that the cars are show ready, or is it a fleet, they want them cleaned, with maybe a detail from time to time?
The absolute fastest way I have washed a car is to have a guy that has a wash mit or pad ahead of me, I have a regular rinse, then someone behind vacuuming. Hit the car with the mitt, with whatever car wash soap you use, and he goes to the next car. You rinse it, and the third guy vacuums, and hits the dash. At the end of the wash, you go back, and quick rinse the windows again, with DI water, while the two guys are finishing the interiors. You might get a little water spotting on the windows, but really, it does a great job. Like I said, how detail oriented are they?