You're correct that a quicker overall response time can be had by getting out the door quicker, and not by racing to the scene. The two minute en-route requirement is fair, given that people can be responding from a dead sleep, PT, needing to turn off the stove and oven while cooking, going number two, etc. You also have the old timers that stiffen up while asleep, and need to loosen up just to get into their pants. You have people that are working mandatory OT, and may be on the tail end of a busy 48 hour shift, and need to have their bed kicked a few times to get them up.
Normally, during the day, the en-route flex time is typically 30 seconds to a minute if nothing significant is going on like a drill or PT.
Also, regarding your last post, if we're on the ambulance, and you leave me behind, you're going to be responding by yourself. If you're on a suppression piece, you're going against safe staffing standards, as evidenced by the recent NIST studies (which I've participated in).