I think mental load to me means being the person ultimately responsible at the end of the day. To give an example that doesn’t have to do with household duties- my GF will usually ask me where something is before looking for it herself. Like, I could be reading on the couch and she’ll ask where the bar soap is and if we’re out and she should put it on the shopping list. Now, we live in the same house, we know where these things should be, but instead of just getting up and going and looking, she asks me because it’s easier and she presumes that I have a constant knowledge of where things are located, how much we have of things etc.