May 11, 2010
Speed Up Your Site
The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won’t start any other downloads, even on different hostnames.
The page is quite comprehensive and explains the basics, e.g. minimizing the number of HTTP requests, and some more nuanced techniques, e.g. not using CSS expressions because of the number of times they are evaluated within a browser.
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