A while ago, Fabien asked me to have a look at porting one of Twig's slowest methods, TwigTemplate::getAttribute(), into a PHP extension. It is a complex method that does a lot of different checks and look-ups. Fabien's benchmarks showed that this method was responsible for quite a large amount of time. On top of that, it didn't seem that it could be optimised any further as PHP code itself.
He points to the twig-ext extension that's a reworked version of the "getAttribute" method from the tool and the performance gain (about 15%) it gives. Compiled templates will automatically call this new method in the extension. This update has already been merged into the main Twig repo.