This is a common problem on most GM vehicles made in the 2000's, possibly the 90's, and should almost be considered planned maintenance because it WILL HAPPEN. Your car will take a while to start, either all of a sudden, or progressively longer over time depending on how quickly your FUEL PRESSURE REGULATOR is failing. What happens is a little rubbery diaphragm inside of the pressure regulator that moves back and forth all of its life gets a hole or a tear in it, either from fuel choice or simply wearing out. This causes incorrect fuel deliver during start up, but once the juices are flowing the car may smooth out and run perfectly in most cases. It is worth looking into if anyone ever tells you your fuel pump is going out because this part only costs about $50 on a bad day. Typical life expectancy for this part is a little over 100,000 miles. Mine made it for 160,000. Some go sooner as well.