Use the web server module instead of the CGI. That eliminates a lot of headaches and makes things load much faster. The SquirrelMail IMAP functions were benched with both versions and the CGI version was 30% slower than the web server module (on that system).
PHP was written to be extendable with loadable modules. With PHP 4, Zend has built-in some additional code to make PHP load faster. That's why with a phpinfo() without the Zend Optimizer enabled, you still see a message about Zend near the top of the screen.
The use of a [PHP cache] might speed up things further.