The picture says it all. Multitasking really slows you down. Suppose you have two tasks to do: Task A and Task B. When you perform your tasks alternately (as in the top line) you pay a big overhead cost. Each time you are resuming a task, you have to load all the information related to it into your head and to enter into the state of mind that you had been when you last stopped doing it. This overhead is known in computer science as context switch.
When you work on each task until it is done (bottom line), you save yourself the context switch overhead. And therefore you are able to finish earlier. Each of the tasks on the bottom line is completed before the corresponding task on the first row. By the deadline (finish time), you had completed both tasks and even had time to rest (the blue arrow).
I prefer to manage my work as in the second row. When I work on a software project, I always get into my working room knowing that I am on the task until it is done. After several minutes, my mind enters the world of the project I am doing. I can’t hear or see anything not related to it. I totally live it.
And before I notice, the task is done.
So why do people multitask?
Sometimes there is a need to show progress in two tasks. In that case, you can’t avoid switching between them. Sometimes you just can’t progress on a task until you get additional inputs on it and then it makes sense to switch to the other one.
Some people choose to multitask whenever one of the tasks becomes boring or they just don’t want to do it. It is like doing it in very small steps, where each step is bearable. But then, they spend on each and every step the overhead of switching to the task that they don’t enjoy doing. Quite weird, since with this strategy they end up spending more time on the things they don’t like. The solution is obvious. Allocate enough time and finish with the things you don’t enjoy doing at once!