As others have said in answer, it's absolutely the fault of the various Flash embedded players. It's somewhat easy to prove if you write your own player (not i

As others have said in answer, it's absolutely the fault of the various Flash embedded players. It's somewhat easy to prove if you write your own player (not in Flash) to bypass the Flash player of sites like YouTube, Twitch, etc. The experience becomes perfectly smooth (as long as you have the average bandwidth), the video starts quickly, jumping to another point in the video is almost instant.