So every time you switch, there is a period of adjustment as your reorganize your thoughts and have to re-focus on the new target task.
Because it can take a few minutes to get up to speed on a task, this switching cost makes multitasking extremely inefficient!
Of course this depends on the type of tasks you are trying to blend. It is possible to listen to an audio book as you clean the car for example. So in certain situations multitasking is ok. BUT this is only when you are not employing the same mental resources.
Writing an email while trying to follow the words in a podcast, both tasks are competing for your attention, focus and language resources, the work will slow down and quality will suffer.
Eliminating multitasking is one of the most productive habits you can develop!
Avoiding multitasking can be as simple as closing the tab with your email and muting your phones notifications. For most jobs, waiting an extra half hour to respond to an email won’t be the end of the world.