Offload complex dynamic requests to another apache (or another machine entirely)
A tremendous amount of effort has gone into optimizing Apache to make it rip files from the filesystem and throw them at incoming http requests as quickly as possible. Unfortunately, sites that solely serve content that sits in files within the filesystem are not typically very popular sites. The huge demand for interactive content has given birth to many projects that are specifically designed to give the end user a highly customizable, dynamic browsing experience.
Unfortunately, as the interactivity of a web site increases, performance typically goes out the window. Interactive content demands a programming language, and interpreting arbitrary code is very expensive (compared to serving static files). One classic "worst case" example is a Perl CGI that makes database requests and sends email by spawning an external sendmail. Compared to the time it takes to serve a static file, a single CGI request of that sort takes an eternity (and many requests therefore take many eternities, bringing your entire server to a crawl).
Apache modules such as mod_perl and mod_php (along with a large helping of programming sanity) can significantly alleviate the pain of spawning external processes on every hit. But even with interpreters built into Apache itself, the rate of serving dynamic requests rarely approaches that of a static web server. One reason for this is ...