Connection Speed

• A common reason for YouTube videos to lag is a low connection speed, meaning that information travels very slowly from YouTube's servers to your computer. You can test to see if a low connection speed is the problem by running a speed test, which you can do for free on numerous websites. YouTube experiences problems at connection speeds under 500 Kbps. If you have dial-up Internet or low-speed broadband, your Internet connection may not be fast enough for YouTube. If you are paying for a higher-speed broadband connection, however, talk to your Internet service provider to find out why your connection speed is slower than you're paying for.
  
  

Incomplete Download

• A YouTube video might lag if there were problems downloading a copy of the video to your cache -- where your Web browser temporarily stores Web pages for you to view. This might be your problem if you're having trouble viewing only one YouTube video, not many of them. You can sometimes solve the problem by reloading the page and restarting the video. If that doesn't work, try clearing your cache. How you do this depends on the browser; in Mozilla Firefox and Internet Explorer, you can find the option under the "Tools" menu.
  
  

High Traffic

• High traffic on YouTube's website can cause videos to lag. This problem occurs because too many people are trying to watch videos at the same time and YouTube is trying to send data to all of them. If the problem doesn't seem to be on your end, the best solution might be to wait and try watching the video later.
  
  

Pausing the Video

• No matter why a YouTube video is lagging, you can usually get around the problem by pausing the video and letting it load completely before you watch it. The loading time will be slow, since lag occurs when the video tries to play faster than it can load. You will, however, be able to watch the whole video once it's finished loading.