I think you've got the charts backwards. Symfony2 is slower than CI in every chart.

It's a matter of cache, APC cache, memcached, "compiled" autoload cache, there are many levels of caching in symfony, and boom, you have speed because many of your hits are hitting the ram memory.

Symfony caches everything , dumping resources ( configs , templates , ... ) into PHP files. So when you are using Twig on a production server you are not actually reading twig files but basic PHP files with echo statements.