If the person taking the test doesn't push themselves to go quickly or worse, drags their feet the entire time, the score will be lower, sometimes far lower, than if they had.
One of my kids was like this. He had us all convinced that he had a processing speed deficit that was getting worse over time. He was also homeschooled, and my impression of his processing speed was that it was sometimes lightning fast and sometimes dismally slow--and tests of processing speed seemed to be designed to bring out his dismally slow side.
Fast forward to his high school years, where he was able to finish the SAT in half the time and still get all (or almost all) of the questions right.
But more generally, when you use test results to infer something essential about a person (intelligence, for example), there is an implicit assumption that the person was trying to do their best when taking the test.
