YouTube Feather, the lite version of YouTube is for users with limited bandwidth slow internet connections and under-powered computers. The feature strips of unnecessary elements from being downloaded, leaving way for the most essential ones
The “Feather” project is intended to serve YouTube video watch pages with the lowest latency possible. It achieves this by severely limiting the features available to the viewer and making use of advanced web techniques for reducing the total amount of bytes downloaded by the browser.
Therefore the video buffering is smooth even on a PC with slow internet connection. This feature is in beta, and you can opt-in for the feather experience