I've seen studies that show that the optimal learning rate occurs when you are 70-75% likely to succeed in a challenge. If it is much easier you tend to downgrade your effort, which leads to learning to half ass at worst, and slow learning at best. If it is much harder the stress of the challenge actually inhibits learning. The exact optimum is thought to be person dependent as a lot has to do with how you respond to challenges, but for the average person 70% effort is the sweet spot.
As I said in a parallel thread, if you're going for "most number of facts you can recite per hour of time spent studying", I can well believe flashcards with a 70% failure rate are "optimum". But if you're trying to have a conversation, watch a movie, or read a newspaper article, and there's a 30% chance you're not going to recognize any given word, you're going to have a hard time.
What you really want is an appropriate level of difficulty for an entire thing you're trying to understand. This could be either because you have one completely new word per paragraph, or because you have 5 moderately difficult words, or 10 not-too-hard words. The fact that the rest of the words might already be super easy for you doesn't mean you aren't still reinforcing them.
That's basically what my algorithm is trying to do: hand you something to read (a sentence, paragraph, section, chapter, whatever) that's at the "right level" of effort for you.
