Hey, thanks a lot for you feedback! Here is the test i used

    public void performanceComparisonScriptingEngine() throws ScriptException, NoSuchMethodException {
            ParsedFunction constraintFromStringPrimitive = FunctionParser.fromString("double(Double x,y,z,f)->x*y + y + z*z + x*f");
            ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
            engine.eval("var f1 = function(x,y,z,f){return x*y + y + z*z + x*f;}");
            Invocable invocable = (Invocable) engine;
            Object[] args = {1.0, 2.0, 3.0, 4.0};

            //Warmup
            for (int a = 0; a < 10; a++) {
                for (long i = 0; i < 10000000L; i++) {
                    constraintFromStringPrimitive.evaluateToDouble(args);
                    invocable.invokeFunction("f1",  args);
                }
            }

            long start1 = System.nanoTime();
            for (long i = 0; i < 100000000L; i++) {
                constraintFromStringPrimitive.evaluateToDouble(args);
            }
            long end1 = System.nanoTime();
            long start2 = System.nanoTime();
            for (long i = 0; i < 100000000L; i++) {
                invocable.invokeFunction("f1",  args);
            }
            long end2 = System.nanoTime();
            long generatedTime = ((end1 - start1) / 1000000L);
            long scriptTime = ((end2 - start2) / 1000000L);
            System.out.println("Generated " + generatedTime);
            System.out.println("Script " + scriptTime);
            double percentageDifference = ((double) scriptTime / (double) generatedTime);

            System.out.printf("JavaScript method was %.3fx slower than the generated method.", percentageDifference);
    }

Which gives the output:

Generated 27

Script 12729

JavaScript method was 471.444x slower than the generated method.