Since 2012, YouTube has been trying to reduce dramatically the time it takes for a video to start from the moment you press play. Flash Networks (Mobixell at the time) was among the first to detect a new proprietary implementation called sliced bread.
The matter might seem trivial, but internal research from Google show that most users find a waiting time exceeding 200ms unacceptable for short videos.
YouTube has been developing a proprietary protocol, based on HTTP adaptive streaming DASH to decrease latency and start time for its videos.