My daughter is a very slow test taker. She knows all of the materials, but simply doesn't move very fast. She is very deliberate over every question, and double and triple-checks everything. She's more than a bit OCD about it, and we have tried to get her to move faster, but it's just the way she's wired.
She's borderline on the autistic spectrum (her brother is fully on it, on the mild side) but not diagnosed and doesn't qualify for an IEP. She does have an ILP for a mild speech disorder and for the gifted program, but untimed testing was denied.
She was unable to finish her AP exams, and didn't even manage to finish some of her final exams this year. What she finished, she got good scores on, but lost two letter grades due to all the blank questions on her finals.