To be really good at something, you probably need:
1. Natural ability
2. Practice (Experience)
but also,
3. Desire to be good.
If you don't have 3, no matter how much 1 and 2 you have, you'll probably pretty average.
As a CSD, I don't think I had a lot of 1., 15 years ago, when I got it. I did however, have a lot of 3., so after a few years of 2. (probably 5-10, if I'm honest) I think/hope I got good at my job. My annual appraisals would suggest I have.
My point is, that some people with a lot of natuaral ability will be a good CSM/CSD/PSR almost straight away, others will need longer, but the one thing that will always prevent exceptional performance is lack of motivation. The old phrase "you never stop learning", with the proviso that "so long as you want to".
Experience is great, because it presents you with a wider range of situations to deal with and to store in the memory banks, so long as you are willing to learn from your mistakes and are not just repeating them time and again.