I believe the higher you go, the slower it adjusts. So starting at Beginner, it goes up to Rookie almost immediately if you get a couple hits, then will take a little longer to get to Rookie+, and longer still to bump up to Veteran, etc.
I've adjusted the Dynamic Difficulty slider, I can't say for certain that has much impact other than if you move it all the way to the Left (0), it does indeed lock in the Difficulty so it won't change. Moving it all the way to the right (10), I can't say the rate that the difficulty changes is noticeably different. I'm on Veteran+ for hitting w/zone and All Star for pitching w/classic right now.