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. Seeing that it was very likely that this method would be a lot faster when written in C, SensioLabs decided to sponser me for the development of a port of this method into a PHP extension.
Over the next few months I've worked on re-implementing just this method as a C-function in twig-ext: twig_template_get_attributes(). From initial benchmarks it looks like this function as implemented in the extension gives about a 15% speed increase while rendering templates. Twig's compiled templates directly contain a call to this function (as opposed to marshal it through the original TwigTemplate::getAttribute() method) for additional performance. This probably means that you'll have to remove your compiled templates cache while upgrading.
The extension at http://github.com/derickr/twig-ext has now been merged, via http://github.com/derickr/Twig, into http://github.com/fabpot/Twig. The plan is that this new improvement is going to be part of Twig 1.4.
If you are also interested in having some of your PHP code ported into a PHP extension in C, please feel free to contact me.