Biggest thing i learned from doing things quickly is where precision matters and where not. For example if i'm painting the ceiling it doesn't matter if i touch a wall as i will paint that wall later (unless i splat a bunch of it, ofc), but it does matter if i touch the ceiling when i'm paining the wall. Of course painting things quickly brings the risk of spraying paint around, especially when using a roller. Painting things too slowly can also be bad if the paint is highly viscous, like with the paint for outside metal surfaces (railings) as it ends up being obvious where you stopped to detail something else. Or wall paint if you cba to do it in two layers so you mix thicker paint and do it in one.

With windows specifically (and doors and such) you would put the.. paper tape (idk how to translate it) (edit: masking tape) some 2 or more cm wide on the edges so you would have a nice margin of error. With paining fast there is a bigger risk of going over it, and that can be bad (oil based window paint on a wall is really bad, wall paint on a window easily goes away with a wet rag).

In most construction it doesn't matter if you are little off, it was just me being anal.

TL;DR I learned where it matters to go a bit slower and where i can go fast without consequences.

edit: Forgot to answer the question. Yes, i did get better. Mostly because i learned where i can go fast and where i need to pay more attention. Learning to not wave brushes full of paint around like a figure skater also had a bit to do with it. I did also learn that some things come out better by default if you do them fast.