I had pretty much this experience after a stroke that damaged working memory, speech production, and attention... I just could not keep up with the cognitive demands of my pre-existing life, friends, work, etc. I couldn't even read more than a page or so of text at a time.

Unlike the author, I hated it. But it did lead me to identify less with being clever, which isn't a bad thing.

This resonates, though:

I developed much more respect for one of my friends in particular who I always considered slow - it turned out he is much deeper than I thought, I just never had the patience to notice before.

Absolutely... I had this epiphany about several friends of mine, with whom I now have much closer relationships than I did.