To a great extent, the computer’s usefulness for serious reading depends on the user’s strength of character. Distractions abound on most people’s computer screens. The reading speed reported in academic studies does not include delays induced by clicking away from the text to see the new email that just arrived or check out what’s new on your favorite blog. In one study, workers switched tasks about every three minutes and took over 23 minutes on average to return to a task. Frequent task switching costs time and interferes with the concentration needed to think deeply about what you read.
and, a little later, from a different expert:
My own research shows that people are continually distracted when working with digital information. They switch simple activities an average of every three minutes (e.g. reading email or IM) and switch projects about every 10 and a half minutes. It’s just not possible to engage in deep thought about a topic when we’re switching so rapidly.
Ain't that the truth! I have multiple desktops on my machine; it's the only way I can stay focused. I bury stuff I'm not using, but need to keep open, on other screens than the one I am working on. Even then...now, what was I about to do? Oh, yeah...