Most of this stuff is not PHP specific, and Python or Ruby or Java or .NET developers can use the tools in this talk.
Slides are at http://talks.php.net/show/confoo10
“My name is Rasmus, I’ve been around for a long time. I’ve been doing this web stuff since 1992/1993.”
“Generally performance is not a PHP problem.” Webservers not config’d, no expire headers on images, no favicon.
Tools: Firefox/Firebug extension called YSlow (developed by yahoo) gives you a grade on your site.