Retailers this holiday season are focusing on delivering packages to customers on specific dates, rather than competing on speed of delivery. The shift marks an easing in a race for delivery speed in e-commerce in recent years that has pushed goods to shoppers’ homes at an ever-faster pace while narrowing retailers’ profit margins on sales. With inflation-conscious consumers now dialing back their online shopping, many retailers are focused on restraining the high costs of fulfillment and last-mile delivery. Amazon.com Inc. set the pace for competition on speed with its investment in next-day and same-day delivery. In 2019, the e-commerce giant...