I'm really not seeing either of your examples, unfortunately. What's stopping average fitness person from noticing their times aren't as good in the same way olympian-level person would notice that their times aren't as good? What's stopping him from noticing his morning jog or whatever is tougher?
Why wouldn't average reader have more difficulty parsing badly punctuated writing as well? Why wouldn't they be able to parse it in different plausible ways too?
I'm just not seeing it.
Edit: To go into more detail:
Person A, B and C are secretly transplanted into a low oxygen universe.
Person A noticed their 200m backstroke time is consistently 4 seconds slower than usual.
Person B notices they have to take walking breaks more often than usual on their morning jog, and took 5 minutes extra to complete their usual route.
Person C has more difficulty doing basic tasks.
Person D, E and F are each given 10 badly punctuated sentences to read.
Person D finds they can parse 4 of them in different plausible ways.
Person E finds they can parse 2 of them in different plausible ways, and can't parse 2 of them at all.
Person F can't parse 4 of them at all.
I see these examples as being just as plausible if not more than yours.
I imagine person B as someone who doesn't do formal exercise at all. If their capacity for exercise goes down, they might think vaguely that they're a little sick or getting older, but they've never been concerned with the details of how much they can do, and it's going to a good-sized change for them to notice it.
However, I'm not sure this pattern extends to reading-- some smart people who read easily seem to have the metaphorical proof-reading/copy-editing gene.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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