This option has disappeared from YouTube. We have kept the page for informative purposes.
If you're dealing with a slow internet connection, loading high-quality YouTube videos is a nightmare — but it doesn't have to be. You can improve your video-watching experience by disabling the playback of HD videos. At the risk of a lower quality image, your videos will load much faster.
Configure Playback Options on YouTube
Sign in to YouTube, and click Settings in the My Account section.
Go to the Playback Setup section. Under Video Playback Quality, check I have a slow connection. Never play higher-quality video.