YouTube's goal is to help you compare the video speed from your ISP with similar numbers from other ISPs. "Video speed is an important part of your viewing experience since it determines the amount of time you need to wait before you can start watching a video. It is also an important factor in determining the quality of the video you can watch. By making the video speed history data available, we hope to better inform you of speed issues as they relate to your viewing experience, and give you the ability to compare your speed numbers with other users in your region," mentions YouTube.
Average speed in the US = 3.62 Mbps
Average speed in Japan = 5.53 Mbps
Wired thinks that "this level of transparency could pressure ISPs not to slow down data from YouTube, because it will be clear to consumers if a given provider offers a poor connection". It's also a great way to compare ISPs and to test your Internet connection, but you should take the results with a grain of salt because the speed numbers are only for YouTube video traffic.
Tip. Some YouTube videos have an additional option in the contextual menu: "show video info". This feature shows information about the video and some streaming stats.
