On the PHP::Impact blog there's a recent post that details a statistic Paul Jones found with the Zend Framework's performance - a drop in the response time of the front controller between the 1.0 and 1.5 releases.

The most important factor in making a Web application fast is its basic design. You must also know what kinds of processing your framework is doing, and what its bottlenecks are. The best way to find the performance bottlenecks is to monitor the performance counters and to have a thorough understanding of the framework your application is using.

Federico runs some benchmarks of his own (using the Apache benchmarking tool, ab) both with a default "base app" and with a more optimized "take out what you don't need" version.

Since there was such a good response to his previous framework benchmarking post, Paul Jones has come up with a fresh batch of statistics for the latest versions of several frameworks including the Zend Framework, Solar and CakePHP.

By popular request, here is an update of my web framework benchmarks report. [...] In the interest of putting to rest any accusations of bias or favoritism, the entire project codebase is available for public review and criticism here.

Follows the same methods as before, creating "hello world" controllers in each and running the stats against them with the Apache ab tool. A baseline non-framework example is created and tests are run against different versions (including release candidates) of each framework.

I won't spoil the results - you'll just need to check out his post to see who ranked where.