Earlier quoted context omitted.
The research suggests SAT test prep has an effect of about one seventh of a standard deviation – about 50 points out of 2400[1]. That's significant but not huge. Most of the perception of large gains comes from the fact that people do better as they get older with or without coaching. (None of the research is great, but better than nothing.) [1]: https://www.jefftk.com/p/sat-coaching-what-effect-size
The College Board released new results in 2017 showing a more than 50 point increase out of 1600 versus students who didn't practice. This was with just 20 hours of practice. Many high achieving students practice for 5+ hours per week. South Korean cram school style 2-3 hours per day is in a completely different league than a standard SAT prep course, but much of the research that shows no result counts them the same…
This is like pointing out that I can make large gains in my 100m sprint time by practicing for half an hour a day for a month and predicting that I can make the Olympic team if I put in a thousand hours practice. Novice gains are real and significant but they peter out pretty quickly.