YouTube has a global network. There are multiple copies of YouTube videos in their servers all around the world, so they can start the video stream from as close to you as possible.
Here is a useful tool to evaluate you can use to evaluate your YouTube HD playback. There are many factors that influence your video streaming quality, including your choice of Internet Service Provider (ISP). Learn how your ISP performs and understand your options.Users on YouTube HD Verified networks should expect smooth playback most of the time when watching high-definition YouTube videos (720p and above).
Click here to test your network