Performance is an issue often mentioned by developers. Regarding zend framework, one consideration is using Zend autoload or not. Autoload can make our life much easier. As long as we specify the file name and class name in accordance with name space rule, we don't need to put include/require on top of files. Some articles on internet, however, point out that zend autoload slow down the web site's performance. They prefer puting lot of include/require statement on top of files.
For me, i prefer using autoload, because not only it really saves a lot of time, and makes the code look more clean, but also most problems of a web application, based on my opinions, are caused by coding errors, bugs instead of performance.