"Feather" Beta is an opt-in feature of YouTube, and is intended “to serve YouTube video watch pages with lowest latency possible. It achieves this by severely limiting features available to viewer and making use of advanced web techniques for reducing total amount of bytes downloaded by a browser.” Chrome dev tools show noticeable improvements: resources downloaded when you watch video only have 52 KB, instead of 391 KB. There’re many missing features in "feather": search suggestions, posting comments, viewing all comments, rating videos, customizing embedded player. Another issue's that lite version isn't available for all videos. Its a work in progress and may not work for all videos.
More info: "Feather" Beta