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Parallel Downloads

The biggest impact on response time is the number of components in the page. Each component generates an HTTP request when the cache is empty, and sometimes even when the cache is primed. Knowing that the browser performs HTTP requests in parallel, you may ask why the number of HTTP requests affects response time. Can’t the browser download them all at once?

The explanation goes back to the HTTP/1.1 specification, which suggests that browsers download two components in parallel per hostname (http://www.w3.org/protocols/rfc2616/rfc2616-sec8.html#sec8.1.4). Many web pages download all their components from a single hostname. Viewing these HTTP requests reveals a stair-step pattern, as shown in Figure 8-1.


  

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