Mark Bauerlein has an article on the difficulty of slow reading online. A taste:

In the eye-tracking test, only one in six subjects read Web pages linearly, sentence by sentence. The rest jumped around chasing keywords, bullet points, visuals, and color and typeface variations. In another experiment on how people read e-newsletters, informational e-mail messages, and news feeds, Nielsen exclaimed, "'Reading' is not even the right word." The subjects usually read only the first two words in headlines, and they ignored the introductory sections. They wanted the "nut" and nothing else. A 2003 Nielsen warning asserted that a PDF file strikes users as a "content blob," and they won't read it unless they print it out. A "booklike" page on screen, it seems, turns them off and sends them away.