The Lost Art of Doing One Thing at a Time

run two demanding tasks in parallel. It switches between them, fast, and pays a toll each time it crosses over.
What switching actually costs

Psychologists call it the switch cost: the small lag and error spike that occur whenever attention is yanked from one task to another. Each jump is brief, sometimes a fraction of a second. The problem is volume. Across a workday of constant pinging, those fractions stack into hours of foggy, half-present effort. You feel busy and finish little, and the fatigue at the end of the day is real even though the output is thin.

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