I'm pretty sure it is 50fps assuming a good connection. What I have found is that with Chromecast you must force it to 50fps or it is very juddery. I still believe that the stream is being over-compressed so what we are seeing is artefacts from compression. Watch the flight of the ball in front of complex backgrounds, even as a player moves in front of the crowd. Comparatively, in 'simple' situations the picture is quite good.