> In my experience, this is completely wrong. I always strain more to read gray text on white background, no exception.
I can't know for sure, but it's pretty likely your experience and your observations aren't quite aligned. If the contrast is too low (such that you notice), it can certainly cause more strain. But it's often the case that almost-black registers as black (and almost-white as white) and you don't even realize that's what you're seeing.
That's the entire problem with this theory. The designer does NOT know what my computer screen renders. Not every computer is a Mac with a 400+ nits display or has the same contrast ratio. The optimal "gray" therefore varies across screens.
In my case, anything above #333 is capable of producing problems, the range #333 to approx. #444 is problematic with thinner font weights and anything above #444 is easily, noticeably terrible with anything except heading weights.
I can't know for sure, but it's pretty likely your experience and your observations aren't quite aligned. If the contrast is too low (such that you notice), it can certainly cause more strain. But it's often the case that almost-black registers as black (and almost-white as white) and you don't even realize that's what you're seeing.