A very interesting talk: https://www.youtube.com/watch?v=FKTxC9pl-WM I haven’t cross-checked the scientific claims so consider it “provisional.”
Summary: You have a finite, total amount of mental energy and it is fungible. That is, what you use up on one mental task is no longer available for another, completely different mental task. And “mental task” include paying attention to whatever. Therefore, husband your attention with care.
It confirms the efficacy of something I have practiced for a long time