A few weeks ago, a reader noticed an interesting YouTube experiment that displayed a red progress bar when loading video pages. It was obvious that video pages were loaded dynamically using AJAX.
Gizmodo posted adaptive streaming and downloads videos in slices. The first slice of the next video you’re likely to watch is prefetched. This works well for playlists and search results (YouTube prefetches the first result), but also for video pages, where YouTube shows suggestions.
YouTube hopes to reduce the time between clicking a video and playing the video to less 200 milliseconds, which is what the human mind can perceive as a delay. That means instant video playing (if you ignore video ads).