Some things do need repetition. Kids thrive on repeating what they love to do - just watch them doing skateboard tricks or singing silly songs until they drive their parents up the wall - and that is more effective than the "brain crushing" method you mention. If your theory is right, you did learn spelling through repetition: you were repeatedly reading and writing words. It just didn't feel repetitive to you because you were interested. Research into performance in my favourite hobby, chess, shows that spatial ability, mathematical talent, and overall IQ are all irrelevant in predicting whether a kid will grow up to be a chess superstar. The main predictor of whether a kid will be brilliant at chess is how much she loves playing chess! The more she loves it, the more she practices and the more she practices, the better she gets.
One point which no one has yet mentioned is that repetition is often used at school as a substitute for waiting for a child to be developmentally ready. There are many things which a child could either master by doing masses of repetition at the age of five, or by waiting until the age of ten and simply learning it fairly quickly and effortlessly. (Do you know the adage, "Start toilet training at two and finish at three, or start at three and finish at three"?)
I sometimes tell parents of young children who come to me to learn chess that they mustn't be disappointed if their child seems to take a long time to get the hang of ideas which seem obvious to the parent. Children aged 4+ are generally capable of learning all the same chess concepts as older kids and adults, even very advanced concepts. It just takes young kids much more repetition to get them. That doesn't mean they shouldn't play chess if they are interested, but there is no particular benefit to starting young and if the child isn't keen, I don't recommend it as they can get discouraged by their slow progress.
I can give several striking examples from my older daughter's experience. She was autonomously educated and undertook all sorts of things later than most kids would. I am sure she has no particular talent for these things; she just waited until she was ready. Last week, aged 15, she started group music lessons alongside much younger kids, most of them aged 7-9. The teacher mentioned to her that in his experience she'd likely progress far faster than the others due to her age, so she should feel free to work ahead. She joined the class late, missing the first three lessons, by which time the others had already reached Lesson 12 in their book. We bought the book on Saturday and by Tuesday she was on Lesson 22. After years of doggy paddling she wanted to learn proper strokes and started swimming lessons at the local pool. She was twelve when she began lessons alongside seven year olds. She progressed through all the swimming levels in two terms.
Motivation? Life experience? Muscle strength and coordination? Brain development? Whatever made it easier for her, her age eliminated the need for so much repetition. It seems unnecessary and unkind to drag an unwilling child to toil over lessons for years if you could wait for her to be willing and ready and do it more easily. Watching the lessons at the pool, I overheard another parent say, "It took three years for my son to reach Level 2. Good thing we started him when he was four. He's never liked it, but it's paying off." This is the same viewpoint which leads schools in this country to require all four year olds to work toward reading and writing, rather than waiting a few years when most of them would pick it up more easily.