Here’s an interesting web framewok benchmark with a huge list of participants:
http://www.techempower.com/benchmarks/#section=data-r4
In general PHP frameworks do not cope very well (symfony2 is even on the last place for peak DB response times) but raw PHP is at least above average. It would be interesting to see Yii’s results there, too. It’s easy to integrate: They host the tests on github and everyone is invited to add more frameworks. Volunteers, anyone? 