Where the returns of doing an action decrease as more is put in.
In mathematical terms, it's like a log-scale.
This is likely one reason why we take breaks or stop after a while.
Attention has diminishing returns.
It applies to almost everything: you start finding easy/visible errors
and have to hunt for the tiny areas you can improve.
For instance, homeless people are happier to get a $1000 money bill than Elon Musk, who wouldn't care one breath.
The examination system also has diminishing returns, but you might actually still need to do it due to the education system... Marks gained will be lowered becuase it's exponentially harder to retain as you know more and more (Anaology: After learning the easy-to-learn marks, you also need to learn the hard-to-learn.) So brute force it, or not.
But the argument that "diminishing returns is still beneficial" creeps up again. In the world of exams it is where the diminishing returns MUST be done to get have a chance of getting from A to A*! There is no 80/20. There is only All and suffering. In an ideal world though perhaps sleeping whenever fatigue hits can boost overall productivity. Though the world is already ruined, The world is not ideal, thus many students burn in a cage, their brains, such that they can get the "treasure" they need, or the entry-level requirement