Paul Jones has run some benchmarking on four different frameworks - CakePHP, Solar, Symfony, and the Zend Framework - to check for overall speed and has come up with some interesting results.
Anger-inducing broad-brush overview: Solar is 4x faster than Zend, and almost 2x faster than Symfony. Read on for all the nuances and caveats.
Throughout the rest of the post he talks about the methods that he took for testing the frameworks and that his goal was the fastest response time only, nothing about the actual functionality. The test was as simple of a "hello world" that he could get in each one.
He's provided the benchmarking results, as provided by Apache's ab testing script for the base (just PHP manually) and then for each of the frameworks, providing the code used as well. There's also a helpful bit on which classes get loaded for each of the frameworks to see what the overhead is. Symfony tops off this list with 43 total libraries for what ends up as a simple echo statement.