Agreed. It doesn't matter how efficient the algorithm is if people get bored and stop using it. At Polly Lingual (https://pollylingu.al) we tried to tune our space-repitition flashcards for this. We aimed for small lessons that move more quickly, with optional follow up for difficult items. For casual learners this doesn't deter them, and for advanced students, they can still go back and see where they need improvement or just simply do the lessons again.