On Previous versions of symfony (lower than 2.6.8) the response time of a page was about 800 ms in dev environment. With the upgrade in 2.6.8 the response time was more than 4s with high usage of CPU. In production environment, I didn't notice any response time change between 2.6.7 and 2.6.8.
The timeline of the toolbar shows an Initialization time from 300ms for 2.6.7 to 3800ms for 2.6.8.
Environment :
Linux Ubuntu 15.04
PHP 5.5.9