Might have been an issue with the follow-on DEN-SFO flight that required them to pack as many people as possible on your plane. And with the flight at 100% load and many of them standbys with just carry-on, I am not surprised overhead bin space ran out and they had to gate check stuff. And since the baggage guys have to climb the steps, grab a bag, run back down and under the plane and drop it on the loader and then repeat, I can see why it took awhile. I imagine they had at least one or two ramp guys doing the baggage loading, but if they had more, that would have sped it up, but chances are some of them were needed for another flight, which slowed down the loading of yours.