Yes I've suffered the same - farsightedness making it hard to focus literally and a slight astigmatism where one eye focuses a little better than the other combine to increase fatigue and make prolonged writing very difficult

With glasses my whole mind relaxes and I can settle right into the zone

Yeah, same here. It turns out I could focus, but it took a lot of brain power to actually and understand text.

Getting new glasses lowered the cognitive load.

Yes, and in my case, it's still hard to read text off a backlit screen, even with the right prescription. I have to use e-ink or paper if I'm going to be reading for a while.