Drucker, in The Effective Executive
“Effective executives know that they have to get many things done — and done effectively. Therefore, they concentrate — their own time and energy as well as that of their organization — on doing one thing at a time, and on doing first things first.”
“This is the ‘secret’ of those who ‘do so many things’ and apparently so many difficult things. They do only one at a time. As a result, they need much less time in the end than the rest of us.”
“The more one can concentrate time, effort, and resources, the greater the number and diversity of tasks one can actually perform.”
Note that: The more you concentrate your efforts, the greater number and diversity of things you can do. Concentration results in getting more done, not less.