Christine Pearson in The New York Times: "Neuroscientists tell us that dividing our attention between competing stimuli instead of handling tasks one at a time actually makes us less efficient..."
Not if you're a writer.
Clarify: not if you're a writer / artist / 'creative' type who spends n% of waking time creating multiple ideas, plots, characters, scenarios - conceptualising stuff, juggling ideas prior to more trad. typing type activities; engaging in activities where procrastination is pretty much a way of life, yet yr constantly under heavy manners of - *cue ominous film-music* duhhh-duhh-duhhhhh - DEADLINES...
In that sort of situation I'd say that multiple concurrent project activities can actually speed things up, since you can play the different projects off against one another - procrastinate on one or more, while working on 'favoured' project/activity; solve problems subconsciously on Project A or B, while doing admin or dull keyboard-bashing stuff on Project C.
As my good friend Skipper Webb always says: "If you want something done, ask a busy man."