Yes - for a certain narrow definition of "task" - but the reality is much more nuanced and comparing brains to single core processors is oversimplifying to the point of inaccuracy. A human brain has tons of "subsystems", and a given task might use some but not all of them. So some combinations of task are perfectly compatible and do not entail performance drop, while others are fairly impossible to do at the same time. Most people have no problem walking and talking at the same time - but talking and typing different things at the same time invariably results in crossed wires.
If I were to offer a tech analogy - the human brain is like an Amiga, with many specialized helper chips coordinated by a central executive which can sequentially multitask but offers no memory isolation between processes...
1. https://www.apa.org/topics/research/multitasking - Not a study. Focuses on productivity (not health, or perceived well-being, supports the idea that the brain have dedicated structures for multi-tasking.
I am aware that there are cognitive loads on some kinds of multi-tasking. That does not translate to all kinds of multi-tasking though.
To say that "the brain is like a computer, single thread" is misleading. There are scenarios in which the brain exceeds in multi-tasking (playing instruments like drums, playing games, etc), and there is plenty of evidence that we're tuned for it in all kinds of ways (but not all of them).
Furthermore, I'm not defending we should multi-task. I just think the metaphor and the "brain is mono thread" idea is both wrong and dumb.
If I were to offer a tech analogy - the human brain is like an Amiga, with many specialized helper chips coordinated by a central executive which can sequentially multitask but offers no memory isolation between processes...