"The screen, for one, seems to encourage more skimming behavior: when we scroll, we tend to read more quickly (and less deeply) than when we move sequentially from page to page. Online, the tendency is compounded as a way of coping with an overload of information. There are so many possible sources, so many pages, so many alternatives to any article or book or document that we read more quickly to compensate. When Ziming Liu, a professor at San Jose State University whose research centers on digital reading and the use of e-books, conducted a review of studies that compared print and digital reading experiences, supplementing their conclusions with his own research, he found that several things had changed. On screen, people tended to browse and scan, to look for keywords, and to read in a less linear, more selective fashion. On the page, they tended to concentrate more on following the text. Skimming, Liu concluded, had become the new reading: the more we read online, the more likely we were to move quickly, without stopping to ponder any one thought."
The New Yorker
This is consistent with my experience and is why I won’t use an e-reader. In fact, I print online pieces of any length to read them in paper-and-ink form.