All SSJS is parsed into an AST (abstract syntax tree) at runtime. In other words, your code just remains a String until the exact moment that it is executed, at which point a parser examines that String to syntactically identify what the code contains: which characters denote variables, which are operators, functions, etc.

This is why SSJS is always slower than the directly equivalent Java: if you just write your code in Java to begin with, then it's compiled into bytecode the moment you build your project, but perhaps more importantly, at runtime it doesn't have to "guess" what code to run by parsing a String... it just runs the Java code you already defined.

But if your goal is optimization, you'll gain far more improvement by moving all code from SSJS blocks to bean methods than you will by eschewing the convenience of @Functions in favor of native JavaScript, because even native JavaScript has to be parsed into an AST.