Attention residue is the idea that there is a cognitive cost to shifting your attention from one task to another.
Sophie Leroy is a business professor at the University of Minnesota. In 2009, she published “Why Is It So Hard to Do My Work?” and introduces the idea of attention residue.
From Deep Work: when you switch from some Task A to another Task B, your attention doesn’t immediately follow—a residue of your attention remains stuck thinking about the original task. This residue gets especially thick if your work on Task A was unbounded and of low intensity before you switched, but even if you finish Task A before moving on, your attention remains divided for a while.