Zend Optimizer is an open-source runtime application used with file scripts encoded by Zend Encoder and Zend Safeguard to boost the overall PHP application runtime speed.
Zend Optimizer decrypts an encrypted code by converting plain text PHP into Zend intermediate code, a binary format. Zend Optimizer interprets the encrypted code to run encoded applications.
Techopedia explains Zend Optimizer
Zend Optimizer requires PHP host server support when running encoded PHP applications. Although the code runs every time a page executes, there is minor but non-noticeable overhead. A supported host server enhances PHP code execution performance by optimizing code, which increases execution speed without changing code.