While this article seems well researched, I find this claim very hard to believe. I can read while singing (not a song with words, but a tune that I know well, without words).
I really don't think I "talk to [myself] so quietly, it cannot be heard" and while my "tongue and vocal chords" may "receive speech signals from [my] brain", I'm pretty sure they're not executed and neither my tongue or my vocal chords move in any way.
(While reading in English there are some words I don't even know how to pronounce, so how could I "subvocalize" them...?)
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The other point I have issues with is the fact that one should retain everything they read while speed reading. While I'm unfamiliar with speed reading classes' claims, I would argue that it's useful to be able to skim a big wall of text to
1- get a rough idea of what it's about
2- determine if it's worth a further, more thorough examination
That may not be called "speed reading", but it's certainly a helpful skill.