Piano players, what is the correct technique in practicing ridiculously fast pieces?
There are a variety of ways to work on achieving a fast tempo, but you always have to remember that it will take time — especially Chopin etudes! My teacher once said that Chopin etudes only feel good after a lifetime of practice. You are doing well with the slow tempo practice. It is always good to try it at tempo once in a while, just to see if you can do it, but I find it helps to increase the metronome speed little by little so that you feel very comfortable with it at each new increment. Practice it several times with each new adjustment. If at the next adjustment it gets sloppy, slow it down again. You can also practice with rhythms. I like doing this for evenness and consistency. Practice it with long-short dotted rhythm, and then short-long. Also, you can try triplet dotted rhythms. Practice a variety of pressures (firm/legato, staccato/leggiero etc.) and dynamics. You may to do some diagnosing of particular problem spots. Perhaps create exercises designed for specific bars th