There's a difference between:
- actively trying to do many tasks at once, switching between them in a matter of few seconds
- starting tasks, not bringing them to completion before starting another task, carrying on part of the second task and the going back to finishing the first.
I think that in the latter case performance doesn't necessarily decrease; in fact, depending on the nature of the second task, it may actually increse (an example would be when I start a translation project, work in the morning, then I go cycling/working out, and in the afternoon I can work better due to higher levels of energy). In the former case, well, probably it's effective only for tasks that do not require a lot of attention and thought.