A deeper understanding of mathematics allows one to mathematically reason much more quickly. This is true at all levels.
This is true. But its converse -- "Reasoning more quickly in mathematics allows for a deeper understanding" -- is false. These timed tests assume the latter because the former is true and more importantly because timed testing is an easily quantifiable, low-cost measure of 'progress' in an education system that currently rewards schools for performance on short-term metrics. So perhaps a basic failure in logic (assuming a converse to be true) is to blame, which is ironic since logic gets such short shrift in the math curriculum!