I've noticed a trend that seems to show that the fuel pumps are starting to get old, and may not be holding rail pressure after the engine is shut off. If the engine starts faster after priming the fuel pump to build pressure, then that would be the cause. Mine definitely starts quicker if I cycle the fuel pump a couple of times before starting, even with a new pump.
Otherwise, I know the ECU on the GM MP4 electronic injection cars doesn't take over from the coil packs until above 700 rpm, so there may be something inherent with the GM coil pack that takes a bit longer.