A time limit identifies knowledge, rather than 'smarts':

An individual who has "mastered the material" can answer quickly irrespective of their smarts: they learnt both the fundamental concepts and the derivatives in preparation for the test, and can commence answering the question immediately from the derivatives.

An individual who has not "mastered the material", but is smart, can start with the fundamentals, work out the derivatives, then commence answering the question: but only given enough time.

So tests which include a time element are, or should be, knowledge tests, and not an intelligence, or 'ability to answer the question', test.