According to Business 2.0, did a site redesign (graphic in pdf) that cut down its average page loading time from 5 seconds to 1.5 seconds and "the number of FedEx customers using an online tool to find out who signed for package deliveries shot up by 835 percent." Pretty impressive testament to the importance of of web site speed. Of course, the uptake could have had more to do with navigation or other changes than download time, but the article doesn't go into this.