In this new post from DevShed today they look at seven different frameworks and compare the speed of how they render a simple request - CakePHP, CodeIgniter, Symfony2, Yii, RainFramework, Doophp and Kohana.

There are so many PHP frameworks that it can be hard to keep track of them all. Some are already very popular, while others are relatively new and unknown. [...] Of the seven criteria [of a good framework], this article will examine only the speed and performance.

They define their structure for testing - hardware and software - as well as some basic settings like number of connections, disabling of caching and these sample files for each of the frameworks. You'll have a see the rest of the post for their results, though.