Quoting Tenni Theurer
30 November 06
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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% (Client-side Code!). 
Tenni’s article is one of the greatest abuses/misuses/bastardizations of the 80/20 rule OR statistics (take your pick) that I have seen. It’s unfounded on so many levels that it’s not worth the time to address.
The conclusion of the article, however, is concurrent with my experience. In most applications, it’s the client-side code that will most greatly affect the speed of the user experience.
@Nate Logan – Wow, calm yourself defender of the 80/20 ruler. I hear chanting can do wonders for the stress brought on by the abuse of the 80/20 rule.