Black Belt Productivity has an interview with Dave Crenshaw, author of The Myth of Multitasking: How “Doing It All” Gets Nothing Done.
Of special note is Crenshaw’s answer to the first question, where he summarizes why multitasking is less efficient. In a nutshell, because it creates switching costs:
I like to use an economics and finance term to describe the waste of time; the term is switching cost. Switching cost usually refers to the cost and time and money of switching from one provider to another. In the case of multitasking, people feel that they are doing multiple things at the same time, but they are not. The brain is incapable of focusing on multiple tasks at the same time. When people attempt to multitask, what they are really doing is switching rapidly back and forth between tasks. Because of this, I prefer to refer to multitasking as switchtasking. It is because of these switches that people lose time in the switching cost. In this way, switchtasking causes us to be exponentially less productive.