As a learning violinist with 2 years experience I can understand this completely, especially with good teaching and focused practice. I noticed a real differen

As a learning violinist with 2 years experience I can understand this completely, especially with good teaching and focused practice. I noticed a real difference between when I switched from a teacher more suited to children to one more for adults. My practice became more focused, my time was better spent and overall the quality of my playing drastically improved. Additionally, if my teacher is away for a week and I go without a lesson, I notice that I can spend a lot of time creating bad habits which then become muscle memory. So I can understand why 10k hours of poor quality practice can be worth 5k hours of good, focused practice