Some of you might be acquainted with Phalcon - "The fastest PHP MVC in the world!!!". The reason it can be so fast is because the core is compiled into a PHP extension rather then being interpreted in real-time (or cached with APC/OPCache). Usually it takes real long time to write extensions and it would really not be feasible. But Phalcon also introduced Zephir, which is a high-level mix of C and PHP that makes it pretty easy to write fast extensions. While learning the language I thought that I would try to make some of the Drupal core functions into a PHP extension just to see how they would perform.