I've been a bookworm from early age and even though I slow down when the material is highly technical and requires better comprehension I don't find the words being repeated in my head. As a matter of fact, while reading I find that while I do talk in my head, I might summarize or ask myself questions about a paragraph (stuff like: "Don't believe. Rem. check refs", like I did a few times in this article), as opposed to repeating the words I'm reading. The only moment I find I'm repeating words mentally or sub-vocalizing is when I'm writing, and I believe I do so to get the underlying tone of what I'm writing.

When I read a novel, I tend to want to spend more time immersing in the world and the events in it. While I slow myself down in this case, I tend to make images in my head while reading and I've found that it becomes inherently harder to do if I'm subvocalizing... it removes focus from the imagination side of things for me. I don't really believe that it's impossible to read without subvocalization.

In any case, one of the tricks I use to speed read at a moderate speed (as opposed to skimming) is to black out mentally everything but the current line and process it as a whole. It works like imagining a rectangle enclosing the current line. Works wonders for me taking into account I'm the fastest reading person in my social circles.