I dont really think mental load necessarily becomes a thing, or a problem until you have kids (or probably caring for an elderly relative with limited capacity) and are then responsible for someone else.
And there become more every day things to anticipate, remember, plan, consider, organise etc. (As much as the action or chore itself) Instead of your own dentist/dr/optician appointments, social life, deadlines, general admin, replacing things that are grown out of or broken, etc etc youre doing that for multiple people. I think it has probably changed for our generation, because more women are working ft. More children are in formal childcare. It's not that each individual action takes ages - they might only take minutes ...but someone has to know when/where/how much/what time/when thats due etc and that takes headspace
I met DH in the supermarket once - we had each gone independently, he was buying ingredients for a recipe he wanted to make that day, I was buying stuff we had run out of, like milk, pet food and things to go in DC lunch boxes. He would have got them "if I'd let him know what we'd run out of...."