Attention Residue is a concept coined by business professor Sophie Leroy. It explains why switching between tasks makes you less effective at both.
When you switch from Task A to Task B, your attention doesn't immediately follow. A residue of your attention remains stuck thinking about the original task. This means when you finally start working on Task B, you are operating with only a fraction of your cognitive capacity.
This is why "quick checks" of your email or Slack are so damaging to deep work. Even if you only spend 30 seconds looking at an unread message, that quick switch forces your brain to load up a completely different context. Then, when you return to your primary task, your attention is divided.
To perform at your peak, you need to work for extended periods on a single task, completely isolated from potential distractions. Batch your shallow work (like emails) into specific time slots, rather than letting it interrupt your deep work.