I own a ereader (now broken) and I read a lot. I did notice that the fact that you have less surface and that turning page takes more time make me read slower. I am waiting for the Entourage Edge in Europe to try with a bigger screen.
I didn't measure the difference in speed but I would say that I did go from 100 p/h to 80. But it's really an estimate. If you read faster it is an handicap.
I would say that when you are looking for a page in a reference book it can be quite fast because you remember where the page is. But I hope that the evolution of interfaces will make a good experience.