My son is/was quite bright - reading at 3, reading the Economist and understanding 20% of it at 5, teaching himself calculus at 7. He got terrible grades in school maths, and his teachers thought he was lazy because he so obviously understood the material.
With some cognitive testing at age 6, he was placed "somewhere over 2nd stdev" (they just stop after a bit) for most cognitive subjects... but when taking response time tests he would drop to 2nd percentile. Second percentile! You could ask him to to find the root of a simple quadratic, and he would think about it and get the answer, then ask him to name the first five even numbers, and he would take about the same amount of time. His processing speed was (and is) just slow. In school, many marks went towards "flash tests" and speed competitions in math. He couldn't get through the first half of the tests, he'd run out of time. He's in third year honours maths at uni now, favourite topic is abstract algebra. They give him more time on tests.
My point is that this is real for some people, it's not just practice or technique or rehearsing.