Polarbear, my DD and I both lose about 5% in speed in digital books, too-- and like your son, both of us are VERY fast readers otherwise.

I think it's the pageturn rate. If you add that up over the course of a book, a difference of 50% in a pageturn really adds up in a hurry.

It gets better if you scale the text size way down (to fit more on a page). Which is a fine solution unless you are over 40. [ahem]

Schrödinger's cat walks into a bar. And doesn't.