Additionally, and this is a point often made by the YSlow guys (I am full of fanboy worship for them since they have made me thousands): Optimizing the black end is black magic full of interventions that you're never quite sure will work until you try them (and sometimes not even then), but optimizing the front end is a science. Front end best practices can be objectively demonstrated to work and they are repeatable.
Example: if your site loads 3 CSS files, and you combine them into 1 CSS file, you will experience performance increases. Period. If you do not gzip your HTML/Javascript/CSS yet, and you are not making a pathological example site just to prove me wrong, you will experience (probably significant!) performance increases. If you increase the number of static assets you load in parallel (for example, by splitting static assets over 2 additional subdomains), you will see most browsers have significant, automatic performance increases for pages that load non-trivial amounts of static assets. etc, etc
Example: if your site loads 3 CSS files, and you combine them into 1 CSS file, you will experience performance increases. Period. If you do not gzip your HTML/Javascript/CSS yet, and you are not making a pathological example site just to prove me wrong, you will experience (probably significant!) performance increases. If you increase the number of static assets you load in parallel (for example, by splitting static assets over 2 additional subdomains), you will see most browsers have significant, automatic performance increases for pages that load non-trivial amounts of static assets. etc, etc