Multitasking is bullshit and not just because it ultimately divides your mental energy by N for every N tasks you try to juggle. Witness how shitty the performance is at both tasks when someone tries to have a text conversation while driving - veering all over the lane and giving trite meaningless responses to the convo.

Multitasking also prevents sufficient interaction across the entirety of your mind that could otherwise bring you over a kind of critical threshold. This threshold is when enough parts of the mind start connecting with each other and all sharing thoughts about the same task that the whole thing just starts to take off. Where the brainpower doesn't just linearly increase, but you start to get so immersed in a task that all your subconscious creativity starts coming out. When mutitasking, you never engage a sufficient percentage of your subminds or let them run for long enough to allow you to get over this critical mass barrier.

There's a big difference between proper scheduling of your time according to specific goals, allowing you to get a lot of shit done effectively, and just throwing a tenth of your dwindling brainpower at ten different tasks at once and half-assing them all.

Side note: there's a cool german term used in mathematics regarding people who have the ablity to put in the hard time of just sitting on their ass at a desk to get the deep work (proofs, papers, etc) done: "sitzfleisch" .