What are some people you feel have had the fastest "growth spurt" in artistic skill?
Has the person ever talked about it?

I always find it interesting how large leaps some people manage to do, so I would love to know more artists like this.

I think Sinix is a good example (even if a part of the "old" internet artist youtubers and subsequent burnout). Pic relateed, was about a year if not a bit less iirc.

He talked about how for a year he just hit a groove with experimenting and constantly drawing whatever he felt like while talking and befriending people from ConceptArt (Workshop and forum).