[super_mario]

I would say your experience is typical for 99% of the people. High school is ridiculously easy even in Europe let alone in North America. High school can be aced with common sense, some intelligence and minimal learning. And if you happen to go to school in a small local community you are doubly unlucky, because it's much easier to be a local star student there and build unrealistic perception of yourself and pick up a lot of horrible habits that will almost grantee you don't make it later in life. You should not ever gauge anything by high school success (high school failure is on the other hand a good predictor of university success).

Then you move to a hard science university and slowly start realizing that you must actually work and study 8 hours a day or more just to keep up and the subject is basically a bottomless pit, there is no end of it and you could spend thousands of lifetimes studying a single subject and still feel like you are only scratching the surface. This is also where intelligence has lesser and lesser significance (almost none). You may notice that good work habits, good organization habits (anything from note taking, listening, attention to detail, to times you study, to light in your study room), and social skills (who you associate with in school is more important then intelligence) and the ability to stay motivated and inspired by your subject etc. matter way more and are a better predictor of academic success.

I had similar feeling as you (I did pure math undergrad and pure math grad school). You are not slowing down mentally, you are just getting fatigued and saturated and getting into more and more esoteric and fringe areas of your subject study. You have to know that most of those visiting professors are lone experts in esoteric razor edge thin subject and they get incredibly excited when they hear there might be another person somewhere across the globe who might have remote interest in roughly the same thing as them. That's just to give you some perspective. I'm sure when you look back at those courses you took years ago and that you thought were incredibly difficult and challenging, you find them trivially easy and boring. That means you have grown by order of magnitude.