Phalcon Zephir vs APC vs HHVM Performance
Zephir can't handle high level objective oriented code. Speed of your application will be worse than clean PHP way.
Here you can find comparsion of clean PHP vs Zephir.https://github.com/dgafka/zephir-BookStore
So, if you want to rewrite your application to zephir, think twice before you will do it.
https://github.com/phalcon/zephir/issues/694#issuecomment-67987616
Zephir is pretty fast (of course, as it is compiled to a C extension)! https://www.simonholywell.com/static/files/2014-02-28/index.html
Hack is pretty fast too, less than Zephir, but on the other hand it's far more mature/active/documented than Zephir.
The main advantage of using Zephir/Hack is that you get static typing as a bonus. It is a real relief to add that kind of security to PHP code.
Dynamic typing has its limits, and permits too much ugly/dangerous things.
Use Zephir for abstract functional coding.
For example, a library of functions that do string formats specific to your project. All advance PHP OOP (business logic) and such should be done within PHP.
Simple as that!
Use the tool that matches the project.