SAT measures your ability to complete the SAT, not your ability to solve math problems. If you are nervous in a testing environment or need to take your time with math problems or write them out beyond the scraps of paper they give you, you will have a bad time on the SAT. Even when I took the GRE for grad school, I ran out of time on the math section. It wasn't difficult, it was just trigonometry and very basic algebra, but I ran out of time anyway. I still got into grad school, so it turned out none of this testing stress mattered at all.