Amazon achieved its “fastest Prime speeds ever” last quarter, the online retail giant announced on Monday. The company says it has delivered more than 1.8 billion units to U.S. Prime members so far this year, nearly four times what it delivered at those speeds by this point in 2019. Across the top 60 largest U.S. metro areas, more than half of Prime member orders arrived the same or next day.

Same-day delivery is currently available on millions of items for customers across more than 90 U.S. metro areas, and Amazon plans to double the number of delivery sites in the coming years.

Amazon notes that the average time from picking a customer’s items to positioning the customer’s package on the outbound dock is 11 minutes in same-day facilities, more than an hour faster than its traditional fulfillment centers. For context, same-day facilities are stocked with what customers in those areas are purchasing, while traditional facilities are larger and include a more vast assortment of items.“As we make these changes, we are seeing that they have the added benefit of reducing costs, too,” Doug Herrington, CEO of Worldwide Amazon Stores, wrote in a blog post. “Regionalizing our network reduces miles traveled and handoffs. Since the beginning of this year alone, the distance between our sites and the customer decreased by 15%, with 12% fewer touchpoints within our middle mile network. Improved product placement gets items even closer to customers, making our delivery system more efficient. And our Same-Day Delivery network is not only our fastest way to get products to customers, it is also one of our lowest cost ways.”

Today’s announcement comes as Amazon has spent the past several years investing in its Prime delivery system and working to make same-day and one-day deliveries the standard.

Earlier this month, Amazon said its two-day Prime sales event this year was the “biggest Prime Day ever.” Across both days, consumers in the U.S. spent $12.7 billon, according to Adobe Analytics data. Adobe says this figure represents 6.1% growth year-over-year, setting a new record for the sales event.The online retail giant says shoppers purchased more than 375 million items during the two-day sale event, up from 300 million items sold last year. Last year, Amazon had two Prime Day sales in July and October, but it’s currently unknown if Amazon plans to do the same this year as well.