Regarding the advice for quieting the voice in your head - yeah that's known as subvocalization. I believe it revolves around going from processing sentences at an individual word (or phonetic level) to processing entire groups of consecutive words at a single time.

I've often felt that speed-reading feels a bit like a parlor trick than an actual useful ability since I find my long-term retention seems to be roughly proportional to the time that I spend quietly internally digesting individual sentences or thoughts before moving onto the next section.

I'd like to see a proper study around short-term and long-term recall with students grouped by their respective WPM reading speeds.