Just to be clear, we are talking about rest breaks, not 25% extra time?
The thing is you can be really clever and have a SEN (its called being twice exceptional). Typically, the high intelligence masks the disability and the disability pulls down the high IQ, so you end up looking sort of average. In this case the SEN is easily overlooked, and the teacher assumes the child is either clever when speaking but lazy in their work, or just a bit average all round. Either way, the child often gets left to struggle on.
Apparently, its frustrating and anxiety-causing for the child because their brain is telling them the answer, but their body is not complying.
DS has a high IQ. I'm not sure how bad his SEN is, but I know it could be worse. However, the 25% extra time, even in his case, is nowhere near enough to compensate for his slow writing speed. There are statistics available that give a range, by age, of how fast DC write, and DS is always off the bottom. For example, if he does 10 words a minute, and everyone else is doing 18-30, then even with extra time, he's only up at 12.5.
(those are just made up numbers but they are within the ballpark of some I was given a few years ago).
For those DC who are really slow, not just DS slow, 25% is just a drop in the ocean.