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.