The Zend Optimizer applies multi pass code optimizations over the code generated by the standard Zend run-time compiler and optimizes the same for faster execution. Although the standard Zend run-time compiler used by PHP is quite fast but Zend Optimizer can make the same run faster by another 40% to 100%.
So, look for servers where Zend Optimizer is pre-installed so that your PHP applications are run more efficiently. Just remember that the encoded files should be compiled with compatible Zend Guard. If you are not aware about the version of the Zend Guard be sure to use the latest version of Zend Optimizer.