On the PHP 10.0 blog, Stas does a little benchmarking of a big part of each request to a Zend Framework application - the Loader.
On Zend Framework lists there was a topic raised about performance impact of Zend_Loader component, which is used for - no surprise here! - loading classes, including autoloading, etc. Some folks thought that since Zend_Loader is executing some code before actual loading the required file, it must cost something. And it makes sense. However, how much does it cost?
He uses a set of scripts he developed to compare the bytecode caches of the requests both with and without the caching turned on. He ran the tests on a PHP 5.2 and PHP 5.3 installs with the lower (better) numbers coming from the latter in requests per second. He highly recommends turning this caching on on your system if you haven't already.