After having lessons for a year now, I do notice my approach in learning new pieces is getting very different. First I wanted to do difficult pieces, but I noticed I learn much less from them. The reason being is that I spend so much time getting the muscle memory, that the real musical lessons, like expression, come far after all that hard work. Something I really never thought about after 6 years without a teacher.

Right now I'm doing Bach Invention 1. I don't find playing it very difficult, compared to the pieces before. But because of that there's much more room to learn different things than just the notes. Suddenly I've to watch my breathing, and breath audibly sharp when legato is breaking for example. The third bar of this piece (Bass Clef) for example, I've to breath (in a note duration) between C > B, between E > G, at the same time breaking the legato to give it more dynamics, and she also says it's quite useful if the audience is watching you, since you prepare them in some way.

I can imagine I could probably learn some complicated pieces. But I doubt I'd get them so well under my muscle memory that I could actually watch those very subtle dynamics other than volume.

That being said, can you relate to this at all? How much time do you actually spend on getting the piece 'under the fingers' versus all the work to bringing it to performance level.

Performance level may take double to triple the time investment of play for the teacher level. Keep in mind that this is a moving target. An amateur is going to tend to sound like an amateur, even if they have prepared.

As skill level increases, the target keeps moving. At the highest levels, studio musicians can go in cold with new sheet music and be ready to record the next minute. A small percentage of pianists can sight read well enough to perform without having practiced the piece recently. However, these are exceptional people. Most amateurs never get close to that level.

Learning a piece is separate from learning to perform. Brute force repetition is a poor way to learn pieces, and a poor way to learn how to perform. Instead, learn the music is in sections, learn how to relax and do mental practice.

What would you define as mental practice?

Mental practice can take many forms. I visualize the sheet music in my head, and hear the music, before I go to sleep at night. This seems to help pinpoint weak spots. Another form of mental practice is to play the piece in your head, knowing where each finger is going to go. Lastly, you can look at the sheet music away from the piano, and mentally play the piece.

As long as you aren't distracted by your own thoughts and fancies, you'll develop. If, like me, you can't stop thinking about all and sundry, then the more difficult the piece, the more problems you have . . . .
Make sure you actually LIKE the piece you're doing!

I like most pieces I'm doing. Though, usually I let my teacher choose the pieces for me. So it's always a surprise. Plus then I don't know how the piece sounds, so aslong I don't search for it I will really read instead of doing it half reading-half by ear.

The last 10% takes 90% of the effort - a cliche, but I have found it to be close to the truth.

Sam

I actually find getting the piece under the fingers harder and much longer than the polishing of it. However my ability to polish a piece is limited and has often been thwarted by boredom once I get into the area of diminishing returns.

A couple of advanced players have said to me it's all about breathing, but I don't want to go there. I can't face being told I have been doing it wrong all these years

There are different levels. Novices might work with the sheet music and hum the main melody. Some might try to write out the chord progression from memory. The more parts of the brain involved, the deeper the connection with the piece.

From what I understand from my teacher and also listening to interviews of concert pianists (like Stephen Hough), professional musicians depend on muscle memory much LESS than amateurs. The reason is that muscle memory can fail you under pressure. To truly memorize of piece, you have to literally be able visualize and write out the score. Not only do your fingers know where to go from motor memory, but you conceptually know exactly what note each finger is hitting on exactly what beat.

Here's a test my teacher taught me:
If you flub up and make a mistake while playing, do you need to "reset" and go back a measure or two? Or can you instantly get back on track and keep going while staying on rhythm? If you have to "reset' then you are depending too much on muscle memory and have more work to do to truly learn and memorize the music.