Multitasking messes with the brain in more than one way. But there is another aspect: the inability to meet deadlines. Multitasking may seem more efficient on the surface, but actually takes more time in the end.
Imagine three tasks A, B and C, that would require 10 minutes each (or 10 hours or 10 days). If they are executed consecutively, the first is complete at H+10, the second at H+20, the third at H +30.
Now imagine that you spend 5 minutes alternately on each task. The result speaks for itself :
You will not save time in completing task C, you will have a 5 minutes (or hours or days) delay in completing task B, and a 10 minutes delay in completing task A.
If you take into account the time required for the individual reconcentration between two tasks, multitasking leads you to a much larger delay.