In a recent study, Jakob Nielsen (no relation to "Nielsen ratings" Nielsen) found that it took it took individuals 6.2% longer to read a story on an iPad and 10.7% longer on a Kindle 2 as compared to reading it in print.
His sample size of 24 users is too small to draw any real conclusions from his data, but I wouldn't be surprised if his findings hold true for tablet-readers at large; I still find myself mesmerized by the page-turning animation in iBooks or sidetracked by an overly forceful flick in Instapaper, whereas books are, you know, just dumb old books.
But we want to know what you think: Are you faster reading the printed page than you are reading on your iPad or Kindle? No one's going to hold it against you—we've been reading paper books for decades where we've only had a few years to train ourselves to do serious reading on tablet devices (Nielsen tested a standard PC monitor, too, which was unsurprisingly the slowest and most loathed of all screens). Still, displays with increasingly high pixel densities, like the iPhone 4's, are making it easier and easier on your eyes to zip through digital text.