Though, I understand that many people, possibly even a majority, read by mentally vocalizing the words at a pace not much faster than speaking them out loud, so they can't actually read substantially faster than they could listen.
I wonder if this is true. I'm a pretty fast reader, so I have to recognize that my experience may not be typical. But I have voice mail with voice to text. The text arrives as an SMS style message. I almost never listen to the voicemail, because it is so very much faster to read. Probably 5x or 10x. It isn't remotely close, but I've never measured it.
Maybe that's why other people aren't so excited to turn on Google Voice for their voicemail with transcription service. If there's no time advantage, the tradeoffs of losing the original voice might not be worth it.
Anyway, to your point about mentally vocalizing: I find that I do this with good fiction books. I read substantially slower when faced with passages containing lots of dialog. But I don't find this happening with most voicemail, email, text messages and the like, even though it is conversational content.