In a new blog post Richard Thomas takes a deeper look (well continues his ) at the performance of the Zend_Loader component of the Zend Framework.

When you use Zend_Loader your files are wrapped in conditional statements. These statements prevent APC and all other cache/accelerator programs to have problems properly caching your file. [...] Now here is the catch-22. Depending on the size of your application and how often different classes are used you can run into problems.

His result doesn't have much to do with the component, though - he points out that conditionals of any type can slow things down in an application and that "benchmarking is your friend".