That's because the Web server is busy and cannot process your request. You may also encounter slowdowns because of network congestion caused by heavy traffic in specific regions. The bad news is that they're expected to increase in frequency and magnitude as the Web continues to grow. The good news is that people like Associate Professor Lixia Zhang in the Department of Computer Science are working on a way to put an end to this problem. Zhang heads the Internet Research Laboratory and is principal investigator on all of its projects.
Problem is, hot spots move around.
With the exception of a few cases such as the release of Special Prosecutor Kenneth Starr's report to Congress, it is impossible to predict exactly when and where these hot spots would occur. Starr's report was simultaneously released to multiple sites in an effort to prevent overloading. It was easy to predict there would be a high level of interest in the Starr Report and plan accordingly. Unfortunately, this is not true of most sites which cause overloading. So attempts to solve the problem by adding resources would require resources to be added everywhere.