Megan McArdle is a natural speed reader. She explains her technique:

I skip things. This may seem obvious, but I actually had to catch myself doing it; it is not a conscious process, and if I think about it, I can’t do it. Somehow, my brain selects chunks of text that it thinks won’t convey new information, and avoids them. Perhaps this is not optimal, but it works well enough for me to have made A’s in most of my college lit classes. I can still read faster than most people while reading completely, and I do for some things, like textbooks, but it takes effort and I don’t enjoy it as much.

I do pretty much the same thing although mostly with non-fiction; I tend to read novels completely. If I’m making an effort to read a book for its own sake, I’m not particularly fast. If, however, I’m doing research and reading to answer specific questions or to inform myself on narrow topics, I can go through several books and articles, picking out just what I need, very quickly.

The same holds true online, especially things I’m reading when searching for things to write about on my blogs. I have a sense of what is “blogworthy” and can quickly skim articles and blog posts to see whether there’s an interesting (to me) hook.