If you expect your website will attract a crowd, performance and optimization issues should be a major factor during the development phase. Rest assured, performance has always been a chief concern among the core symfony developers.
While the advantages gained by accelerating the development process result in some overhead, the core symfony developers have always been cognizant of performance requirements. Accordingly, every class and every method have been closely inspected and optimized to be as fast as possible. The basic overhead, which you can measure by comparing the time to display a "hello, world" message with and without symfony, is minimal. As a result, the framework is scalable and reacts well to stress tests. And as the ultimate proof, some websites with extremely high traffic (that is, websites with millions of active subscribers and a lot of server-pressuring Ajax interactions) use symfony and are very satisfied with its performance. Check the list of websites developed with symfony in the wiki (http://trac.symfony-project.org/wiki/ApplicationsDevelopedWithSymfony) for names.
But, of course, high-traffic websites often have the means to expand the server farm and upgrade hardware as they see fit. If you don't have the resources to do this, or if you want to be sure the full power of the framework is always at your disposal, there are a few tweaks that you can use to further speed up your symfony application. This chapter lists some of the recommended performance optimizations at all levels of the framework and they are mostly for advanced users. Some of them were already mentioned throughout the previous chapters, but you will find it useful to have them all in one place.