More about spaced repetition

 Now we know how the mechanism works, it is easier to predict what is needed in practice. If you have perfect understanding (system 2), you probably need no or little repetition. I know little stories of grandmasters who made progress by use of massive repetitions.

Nobody has learned an encyclopedia by heart by starting with the letter A. Since they became demotivated everytime they needed to know something that started with the letter S or the letter T.

That has to do with the frequency of occurrence. Not only should you start with the most frequent words that start with the letter A, you probably shouldn't start with the letter A at all, since it is not the most frequently used letter. So don't start with "apodictic" while you need to know what "symbiosis" is.

This mechanism translates to learning openings as follows. You don't want to start with the less frequently used variations. Even more, when you need to know 60 variations, you don't want to spend an hour on just one variation, even if it is the most frequently occurring one. Since you probably get one of the 59 other variations in your next game.

You need a solid base of most used variations first, but at the same time you must be aware of motivational issues.

There is a direct relation between understanding and the amount of repetitions needed. Perfect understanding means no or little repetition. But when you have to cover a lot of variations in little time, you can diminish the amount or deepness of understanding by cranking up the amount of repetitions.

So the idea of spaced repetitions is good, but it should be complemented by frequency of occurrence and by your level of understanding. Frequency of occurrence can be influenced by local conditions. I know a chess club where a lot of people play the King's Gambit, and another club where the Benko is very popular. Circumstances that can change overtime.

You need AI for the frequency of occurrence, and you need AI to measure your level of understanding. Both algorithms should be used to present you with the right amount of repetitions at the right time of the right variation.

Furthermore, you need AI to identify your most frequent occurrent weaknesses.