Till has written up a nice (long) post about the performance steps he's taken to make his installations of the Zend Framework run even better than they already are.

The other day our servers were overwhelmed with the rising traffic and I started profiling my application through Xdebug.[...] On this project we currently run with 100,000 visitors per day on average, our peak is Sunday night where we get a ton more traffic than usually. [...] Here are a few things, that helped us. Suggestions are in no particular order and I should add that whatever is applicable for my situation, doesn't have to work for you.

His suggestions include using APC for caching, using __autoload, caching of DB queries, and some Apache tweaks to reduce what it has to load/use for each request.