Obviously, you get your best speeds when you're just accepting the text uncriticially. If you stop to think about it, you won't hit top speeds.
Beyond that, you can certainly train for optimal eye movement, local or global optimum as the case may be. After I took speed-reading, I knew not to look directly at the very ends of lines, and I found it more comfortable to center my eyes on white space than on actual text.
IIRC, I tested up to 800 WPM, but that was on grade-appropriate material in a high school I found easy.
(Of course, this was all in the 1970s, when what one read was black text in a familiar font in neat rows on white paper, all at least somewhat thoughtfully arranged. Things are different now ...)
My best real-life estimates of my reading speed was in the 400 words a minute range, with sub-vocalization. E.g., I have a few times read 3 novels the same day, most memorably 2 days before my PhD qualifying exams. (One day beforehand, I crammed like mad. :D One hour beforehand, I led my fellow students in the most awkward game of frisbee ever.)
But it all depends on the material, and the approach to reading it. E.g., there are certain chapter subsections of Hormander's book on Analysis of Several Complex Variables that famously take a week or more each to "read".
I thought CurtMonash was referring to books nowadays, and I must disagree with him if he's suggesting that typesetting, book design and the quality of book production was better in the 1970s than it is today.