Are you using cURL or VBO as ancillary modules? I had read somewhere that people upgrading to the latest versions of PHP had issues if these modules were not also the latest version.
I notice that you leverage the Zend framework. My suspicion is that you are seeing an improvement in speed due to the modifications the PHP community has done to Zend. When it was first made available in the earlier incarnations of PHP, it was notoriously performance-poor.