Earlier quoted context omitted.
I don't understand, wouldn't it be worse for motivation to take longer to achieve the same results?
What's worse for motivation than taking longer? Boredom? Feeling like what you're doing is low-quality or superficial? Doing something artificial for purely external reasons like grades or exams? Can't speak for anyone else, but for me I would take slower progress over any of these... which makes spaced repetition a hard sell.
Many things. I think HN is a bit of a bubble here, but you'll find a lot of people prefer something enjoyable but slower to something efficient and faster, even if they won't admit it.
See the popularity of Duolingo vs Anki as an example! Or Quizlet vs Anki. Or the scores of students who revise by half-watching dopamine-ified youtube videos rather than doing past papers and flashcards. If you ask people, they'll often say they care for efficiency, but their revealed preferences say otherwise.
Doing large amounts (hours) of Anki day in day out is truly miserable, particularly when the alternatives can be quite enjoyable. And if you burn out before you achieve your goal, is the "efficiency" really worth it vs going slower but eventually getting there?
Plus, a lot of people want to learn e.g. a language because they enjoy the process as well as the end result. Making the process miserable in order to get to the end result faster isn't always a good tradeoff.
Which is what it's about. It's a tradeoff. I'm a big proponent of flashcards, but I think it's important to recognise that you're trading enjoyment for speed in most cases.