This paper talks about how excellent performers come to be, specifically in the context of swimming. The primary takeaway for me was that quantitative improvements in performance result from continuous qualitative improvements compounded over time.
For example, the quantitative improvement for a swimmer might be improving times on a 100m freestyle swim by one second. However, merely swimming for several hours a day won't make them faster. A coach will help them improve on a specific aspect of their stroke, a qualitative improvement that will result in a one second quantitative improvement in performance.
As the authors put it:
Excellence is a qualitative phenomenon. Doing more does not equal doing better. High performers focus on qualitative, not quantitative improvements; it is qualitative improvements which produce significant changes in level of achievement; different levels of achievement really are distinct, and in fact reflect vastly different habits, values, and goals.