It's not just the time spent studying, it's the right kind of studying. "Teaching to the test" so to speak. I liken it to my driver's ed class in high school. My teacher wasn't teaching us how to drive, he was teaching us how to pass the test. Every single thing he taught us was about how to pass the test. I passed on my first try because I was very well trained.

Unfortunately, the whiteboard thing is not as standardized, so it's somewhat harder to teach to the test, but it is not impossible. People waste a lot of time I think when they can memorize a small number of basic algorithms and then learn to map the question given to one of those techniques.

But writing the code with a marker on a board definitely takes practice as well.

The available training materials are good, but they are not great. A focus on shortcuts would be nice. I remember my verbal SAT improving significantly just from the single tip to read the reading comprehension questions before reading the passage. I suspect there are similar tips that can improve your speed at figuring out the trick that solves these interview problems quickly.