Most PHP frameworks are written in PHP. So when a request for a URL comes in to the server, PHP must open and interpret all the needed framework files, in addition to any code you write. This takes a lot of time. Technologies like OPCache help reduce this time.
Phalcon is different, in that it's loaded as a PHP extension. This means it's only loaded once - when the Apache server is started. When a request for a URL comes in to the server, PHP must open and interpret any code you write. However, all the framework (Phalcon) code is already loaded, which means none of the framework files need to be interpreted.
One downside, howere, is that you can't bundle up all the necessary files for your website into a single portable directory. Phalcon has to be installed like other extensions and your code can only run on a server with that extension installed.