No, FormCalc is faster than JavaScript as it has more direct access to the object model and is tuned for doing the things it is able to do whereas JavaScript is a broader more general scripting language.
If you want to improve performance you should review your code and make sure your scripts are as efficient as possible. In particular I recommend using resolveNode() as little as possible.
Depending on what your doing, and how big and complex your form is, 3-5 seconds may not be that bad.
Chris
Adobe Enterprise Developer Support