Don’t tell the kiddies, but Santa’s workshop isn’t at the North Pole. It’s here in Tracy, California, next to a line of rusting, graffiti-covered freight cars, in a building so long that you can’t see the whole thing without turning your head to take in its full immensity. This is Amazon’s latest-generation warehouse, a robot-powered marvel of efficiency that in some ways feels even more improbable than flying reindeer. In early 2012, Amazon spent $775 million to acquire Kiva Systems, a Massachusetts-based startup that makes warehouse robots and software to automate the most mundane parts of filling online orders. For the past two years, the world’s largest online retailer has kept mostly quiet about how it was using the Kiva robots in its own operations. But on Sunday, the day before the high point of the internet retail year, Amazon opened its doors to show off the droids that are taking its warehouses to the next level of consumer gratification. The speed of commerce continues to accelerate, with Amazon setting the pace. The Tracy fulfillment center, about an hour east of San Francisco, opened last year as part of Amazon’s strategy to locate more inventory closer to population hubs. Less distance means quicker delivery, including same-day. The Kiva robots are another part of the plan to sling more goods faster than ever. Speed doesn’t come easy, but if Amazon can stay out in front, it can set the expectations that rivals will have no choice but to try to match.