People like to think they can multitask, but the brain actually can focus on just one thing at a time. When people switch between tasks, their brains can't keep up with everything. So there will be a delay as their attention moves from one task to another. Someone who is listening to one person talk, for instance, can't also listen to another. They can't even listen and read at the same time.
-from https://www.sciencenewsforstudents.org "Phones in the Classroom Hurt Everyone's Grades" Sep 10, 2018