Something we've been trying to cut down on at work is the amount of context switching that the development team have to do. Rather than working on multiple projects at once, we've been trying to get them onto one project that they can concentrate on all day.
The reason we do this is we think there's a cognitive cost of changing what you're doing. It's obvious, and it's not a new observation:
A man commonly saunters a little in turning his hand from one sort of employment to another. When he first begins the new work, he is seldom very keen and hearty; his mind, as they say, does not go to it, and for some time he rather trifles than applies to good purpose. The habit of sauntering, and of indolent careless application, which is naturally, or rather necessarily, acquired by every country workman who is obliged to change his work and his tools every half hour, and to apply his hand in twenty different ways almost every day of his life, renders him almost always slothful and lazy, and incapable of any vigorous application, even on the most pressing occasions. Independent, therefore, of his deficiency in point of dexterity, this cause alone must always reduce considerably the quantity of work which he is capable of performing.
Adam Smith - The Wealth of Nations (1776)
It amazes me when people say "Can't you just work on both things at once?" but it's very common in some workplaces. I guess some things take a few centuries to sink in.