The usage scenario fits my needs, that is the reason to evaluate exactly this 

I mean - should the QScriptEngine be deprecated, the QQmlEngine/QJSEngine 
should also have ways to utilize some form of JIT/preparse. Even in the first 
pass where JIT cannot be effective (or when I parse just the text expression) 
QScriptEngine beats newer and more shiny QJSEngine a lot. That is confusing to 
me and my point - why is QJSEngine offered as replacement of QScriptEngine when 
it is so much slower?

QJSEngine does not cache the compilation results if you just pass in plain 
strings. I also don't think QScriptEngine can do this here (but I haven't 
checked). QScriptEngine might be clever enough to automatically use the 
interpreter rather than JIT-compiling such small expressions. You can set the 
QV4_FORCE_INTERPRETER environment variable to force QJSEngine to use the 
interpreter rather than the JIT.

In any case it's a waste to parse the expressions from strings over and over 
and to not use the JIT. If you care about performance, you should try to parse 
the expressions only once.

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