Additionally, you also want a fair number of problems in any given test to reduce the variance in the grades, and you want the student to be able to finish a significant number exercises that can truly cover the breadth of the content to learn, hopefully with more than one approach as well. If a student takes 2h to solve a problem there is no way they will be able to complete enough of a problem set.
Of course, there are outliers. But personally, especially given my shorter attention span, the ability to do math correctly and quickly was absolutely crucial, and I wouldn't have been able to pass otherwise.
This is a specific example, and the general point is that there a multiple ways of doing things. If a child can avoid 1 step and skip straight to the result more efficiently, that is a valid way of solving a problem.
Many school systems make difference between the ability to solve a problem, and the ability to solve a problem a certain way. Sometimes the first is all that's asked, but when that way of doing things is necessary later, then the second is asked of you.
The point is that you want to prepare the child for what they will need in the future. Sure, perhaps you are doubly exceptional and will be able to adapt on the fly in the future, but you can't design a school system around extremely uncommon students.
I say this as another 2E student that had very similar issues. There is no good way of fixing it except maybe by giving special accommodations to these students. I repeatedly failed exams exactly because I would skip steps this way, but there is no sense fundamentally changing the entire school system and hurting the majority for that.
Because accommodating every kid's needs is expensive, and society is not willing to pay for it.
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.