Multitasking is widely practised and rarely actually possible. What feels like multitasking is task-switching — moving between tasks every few seconds — which costs 20-40% efficiency on each task and produces measurable mental fatigue.
What the brain actually does
True parallel processing is limited to automatic tasks (walking and talking). For anything requiring attention, the brain switches rapidly between tasks. Each switch incurs a cost (the 'switching tax') of attention re-orientation.
What single-tasking unlocks
Sustained focus blocks produce more output in less time than multitasked equivalents. Quality of work improves. Mental fatigue drops. The work feels less hard despite producing more.
Try one day of pure single-tasking: one task at a time, phone out of sight, browser tabs minimised. Most people are surprised by how much more they get done and how much less tired they feel.