The only fair conclusion is that we can't say for sure which device offers the fastest reading speed. But we can say that tablets still haven't beaten the printed book.
After using each device, we asked users to rate their satisfaction on a 1–7 scale, with 7 being the best score. iPad, Kindle, and the printed book all scored fairly high at 5.8, 5.7, and 5.6, respectively. The PC, however, scored an abysmal 3.6.
This study is promising for the future of e-readers and tablet computers. We can expect higher-quality screens in the future, as indicated by the recent release of the iPhone 4 with a 326 dpi display. But even the current generation is almost as good as print in formal performance metrics — and actually scores slightly higher in user satisfaction.
Source: Jakob Nielsen's Alertbox