The biggest secret of efficient and effective multitasking is not doing it.

Working on multiple tasks by switching between one task to another leads to reduced concentration and wasted time due to attention residue.

What is this attention residue?

Attention residue is a part of one activity leaking into the next activity causing reduced concentration. For example, you are taking up an online course on statistics. Your attention is on the course while your phone beeps. You pause the course, quickly answer the text and get back to the course. As soon as you get back, your brain still has a leftover residue from the text message you just answered.

To give you some numbers, let us say you check your phone 10 or more times an hour. 10 times an hour is a pretty common occurrence in the current generation. You would have spent 1 min to check the phone/email/chat, 1 min on the attention residue and 1 min to refocus where you stopped. In total, you end up wasting 30 mins every hour which is a whopping 50% of the time.

Oh, by the way, attention residue will exist even after you complete a task and move to the next task. You cannot avoid that because human brains are wired to think. If you complete a task and move to the next, attention residue occurs only once. If you switch multiple times and then complete a task, not only do you have residue after completion but also after each switch.

Your best bet is to complete the task at hand and then move to the next task. You must aim to reduce distractions and eliminate attention residue as much as possible.