I had a realization when I was learning how to play the guitar – if my practice sessions didn’t have me wincing in pain, I wasn’t making enough progress.
This realization served me well in the 8 months I took lessons as I made faster progress than I had initially expected. The results were evident too – weeks when my practice sessions were more painful always yielded more progress.
The law applies to every skill, of course. The more we feel the pain, the more we’re likely making progress. There is no painless progress.