I've always had the best intentions to get to know The Bulletproof Musician blog. If you don't know it, you owe it to yourself to visit. Here a musician-turned psychologist-turned musician/psychologist posts weekly studies on cognitive and behavioral practices and finds deep and insightful conclusions meant to bring out best practices in performing musicians.
Going through some archives, I was recently intrigued by this post about how easy or how difficult our practicing feels. He’s studying the difference between practicing “to make things easier” and “wanting practice to be easy”. There’s a slight difference: in the former, our practice goal is to start with something hard and progressively make it easier to play; in the latter, we start with something easy and end with something easy, in fact, we avoid difficulties altogether.
The article is based on a study of people playing a simple control game; some were allowed to practice at consistently difficult levels, others were held back. Both groups practiced the same amount of time. Tested at easier levels, both groups performed well. When the stakes were raised, the first group performed significantly better.
The author suggests this is an obvious conclusion (“well duh”). But he’s concerned about the practical application to us as musicians. When we learn a piece, we know what to do, but at some point we plateau (as he says, when we need to get from “good to great”). His suggestion is that things not be perfect before we make our practicing difficult, to avoid plateaus where our practicing is easier. Think about applying this principle to practicing your repertoire, where he’s saying it’s okay to move on before something is perfect as the more we challenge ourselves, the better the finished product:
“Like assigning a set of scales at quarter note = 60, but asking a student to increase the metronome by 2 clicks as soon as they can play it  times in a row with < mistakes. As opposed to simply asking that they play the scales at quarter note = 60, and not increasing the tempo until they’re 100% mistake-free.”
"Modern performers seem to regard their performances as texts rather than acts, and to prepare for them with the same goal as present-day textual editors: to clear away accretions. Not that this is not a laudable and necessary step; but what is an ultimate step for an editor should be only a first step for a performer, as the very temporal relationship between the functions of editing and performing already suggests." -Richard Taruskin, Text and Act