I was (am?) a fairly high iq person, national merit scholarship, 99.8%tile SAT scores, etc. Degree in mathematics, compsci.
I had a stroke in my 30s, and it was thankfully mild (slight dysarthria that went away in a month) but I'm certain it impacted my intelligence and ability to concentrate. I can't focus/concentrate for long sessions anymore. I used to be able study without problem until the need to sleep kicked in, but now I get tired/distracted after only a short time even if I'm well rested. I find complicated math significantly harder to learn than I did prior to the stroke.
When I complained about this to my local PCP he would only run normal post-stroke cognitive tests like word recall, naming places/things, etc. "You test normal".
So I got a 2nd evaluation from a neurologist at Mayo.
Feedback was basically the same thing. He diagnosed mild depression, suggesting that could be part of it, and offered an anti-depression med, also threw in a "you're getting older so can expect some slowdown" for good measure.
I probably should have took it because the clear-to-me-loss of ability, with my doctors not seeming to give a shit, or even talk about it left me with no hope or plan to recover it has made me want to kill myself at times over the years. (I'm fine now, not a threat or risk).
I still read and try and just do my best even though my best isn't what it used to be, and count myself lucky that the stroke didn't leave me disabled or with permanent apparent deficiencies.
I just wish it were easier to find doctors interested in helping you be the best you can instead of simply not broken.