The Short Version
The claim is well-supported by robust scientific evidence. Research from the APA, NIH, Stanford, and peer-reviewed experimental studies consistently shows that what people call "multitasking" — rapidly switching between tasks — imposes measurable cognitive costs, increasing errors and reducing output by an estimated 20–40%. While a tiny fraction (~2.5%) of people may be immune to these effects, and simple compatible tasks may not suffer the same penalties, the claim accurately reflects the strong scientific consensus for the vast majority of real-world work contexts.