I think it’s the weight of cognitive load, it’s like taking up a big space of the RAM in your brain, like trying to do tasks while trying to keep a big number memorized, it greatly reduces your abilities, until you offload it somewhere.

I feel the same feeling when building or refactoring complicated algorithms, where it’s not really possible/worth it to write a test or commit in the middle to offload, so I have to keep a lot in my head, if I do it for longer periods I get a headache