Tony Bibbs has posted a mini-case study about how, at his work, they cut their use of the Zend_Log component of the Zend Framework in half:

As part of the framework we use at work, we borrow what we feel are the best components out there and logging is a key part of that. Logging should be simple to setup, easy to use and should minimize work on the developer. After all, you are going to do a lot of logging, right?

He shows how, with a few changes to how they log (creation of a custom logger), it reduces the number of calls to load a Zend_Registry object each time something needs to be recorded. His code is included in the post along with examples of it in action.