when I had my 2000 black SVT I had the same problem. The Driver side dash vent would stay warm for up to 10 minutes before becomming cool. The middle vent would also be warm but would take less time to cool, the passenger side duct would blow cool air within a few seconds of turning the AC on. I figured it was from the underdash ducting being heat soaked. I would set the blower to high and close the middle and passenger side ducts and allow all the air to flow through the driverside duct and the air would get cold much faster.
The car I have now, 99 SVT, all the ducts blow cold air 15 - 20 seconds after AC is selected on.
