“Most performance optimization today are made on the parts that generate the HTML document (apache, C++, databases, etc.), but those parts only contribute to about 20% of the user’s response time. It’s better to focus on optimizing the parts that contribute to the other 80%.”
Performance-aware web pages design is getting pivotal nowadays and, from a global perspective, the general load the network infrastructures have to handle would decrease significantly if designers kept in mind such issues.
Yahoo! performance researching indeed provided the following – quite predictable though – result: “…90% of the time is spent fetching other components in the page including images, scripts and stylesheets“.
This is, by the way, one of the preminent aspects the omnipresent AJAX technology has impact against, and in my opinion – almost – the only one to justify such enthusiasm: it is well known that asynchronous page refreshing saves a lot of time, and reduces the http server-to-client traffic.
Unfortunately the linked post does not deal with the impact of browser caching, which is very important as well; anyway, the big picture is always the same: websites performances increase as the number of HTTP requests get lower.