"fork" is not a major problem in PHP. In Drupal, every single request compiles a file "common.inc", of several thousands of lines of PHP code. That is what slows it down. It's remarkable that PHP still manages to do that in just 100ms.

In Zend Framework it's slightly better because it includes an autoload option, which implies only PHP files that are actually used by athe request, get loaded. That way the overhead is "only" 50ms.