I have built some big applications with the Yii PHP framework. It's pretty speedy because it "lazy" auto-loads classes. I have no performance complaints.
If you run a PHP opcode cache/accelerator like APC or eAccelerator, and do all of the usual best practices for page loading speed on LAMP setups, then using a PHP framework will be WELL WORTH IT for the development time saved. Performance will not really be an issue unless you are serving an astronomical number of requests!
