Focusing on deepening rather than broadening can have a profound impact. Taking the time to delve deeper into fundamental aspects of piano development and exploring a single piece can lead to overall improvement. By making choices and experimenting, musicians can develop a heightened perspective and control of their craft. This approach saves time and energy while producing noticeable changes within the repertoire.
From the book Piano Playing by Josef Hofmann, the advice he got from his teacher Anton Rubinstein
When we make mistakes, we tend to see them as coincidences while attributing success to our skills. However, the opposite is often true. To improve, identify the problem, repeat challenging sections, observe, listen, and aim for flawless execution. Dedicate focused study sessions for efficient progress.