I can't speak for others, but I'd certainly feel less happy if I knew I was making life choices that were likely to impair my mental function.

Whilst here on this other thread you get existential depression if you're clever:

https://news.ycombinator.com/item?id=12524448

It's hard work being a human being.

I wouldn't mind being slower at mathematics if it meant I could relax on a beach somewhere down south or live in a nice cabin up north.

The brain is good at what you practice it doing. I very much hope that I wouldn't need to be as mentally active as I am right now when I retire.

But if you didn't know, what you feel less happy?

This offshoot reeks strongly of Flowers for Algernon