Your Internet connection may not be sufficient to watch the video stream, or your computer may have insufficient processing power.
If necessary, you can reduce the judder by closing other active programs. For a smooth playback, you need an internet connection with 5 Mbit/s for SD channels and 10 Mbit/s for HD channels. You can check the speed of your connection here.
If the picture on your Apple TV 4 is juddering, set the resolution manually to either 720p50 or 1080p50.