You're right, slower is the right word. I'm very accurate and precise but feel like I am embarrassingly slow. Recollection in particular feels slower, and I don't know if that is because I have much more knowledge to draw upon or because of aging.

What is more definite is that the 'aha-moments' are much less rewarding and because of that I am less attentive in my intellectual pursuits. I have more effective learning strategies but less interest in applying them.

I had a funny experience recently. Gave math lesson to a 15yo. I could sense the higher thoughts frequency in his words and ideas. The kid couldn't think deep, got confused regularly on simple matter, but I could feel how vibrant his spirit was compared to mine. Strange feeling :)

I too feel that intellectual challenges are less rewarding. But average life amongst peers is so dull, I'd rather keep doing that rather than stop. Also I tend to blend this with more altruistic pragmatic goals. Getting old makes your ego less important and so this is a workable compromise.