Matt Knight has a recent post looking at a specific part of the Zend Framework, the Zend_Config component, and how it's overhead can be reduced (and somewhat replaced) using a few lines of PHP 5.3-specific code.

One such limitation that I've recently seen is the performance of Zend_Config. This class underpins the mechanism by which developers provide configuration to the application, and all this config is passed around in the form of Zend_Config objects. [...] For the purposes of this article, I'm referring specifically to config INI files, using Zend_Config_Ini to parse. This is a very common format for Zend applications to use - it is familiar to developers and infrastructure support teams - and is the case where the performance issues become apparent

He talks about parsing ini files and the complications that can come with the Zend Framework method of parsing them - conversion to objects via a recursive method, merging data from different sections and the overhead toArray can cause. He shows an alternative that uses array_replace_recursive to handle the parsing and a class version that offers backwards compatibility with the Zend_Config structure. He claims to have seen a sixty percent jump in performance using these methods over the Zend Framework's defaults.