The 58mbps speed is between your phone and your router, it has nothing to do with the speed between your provider and Youtube's server, and a connection is as fast as the slowest part of the path. (Even a souped up Ferrari can't do more than crawl in a traffic jam.) Buffering just means that the path from you to Youtube is slower (somewhere) that the speed you need to keep the video running. If your speed to your provider os over about 1mbps, the problem is somewhere between your provider and Youtube, and there's nothing anyone can do about it. (The Internet is designed to get data there eventually, not immediately. We're used to a fast enough connection to carry on a video chat in real time, but there's no guarantee about that. But if you send an email to an existing account on a working server, it may take a week to get there, but it will get there.)
