As someone who doesn’t usually subvocalise in any language I speak so long as I know the vocab and the grammar, I have to say that the reading is a bit shallower, but with enough practice you end up catching the key words that tell you whether or not you should pay attention there. I don’t need to read “he said” in my mind every time a male character says something in fiction; I remember the shape of the word and fill it in mentally while I read the rest.
But sometimes I do subvocalise, if I encounter something I can’t fill in, or if there’s something I didn’t understand, or if the writing is especially beautiful. That’s when I want or need to engage more deeply with the text.
A lot of the text in life is really, really inane, and filled with fillers, and honestly there’s no need to engage with it further than just make sure that it’s not something I should be paying attention to. It allows me to spot the things that are important more quickly, and with stronger short-term memory of how things are connected (which is weaker when I subvocalise; I might understand individual sentences better, but remember and understand how what I’m reading now connects to 3 pages ago? If I subvocalised all 3 of those pages I’d 1. become mentally exhausted, 2. take over 15 minutes to read those pages, 3. have an irritated throat, 4. more likely to be distracted by something). If I had to vocalise even the text that’s written on the back of the milk carton (it changes sometimes!), I’d probably go mad with frustration within a week.
So it’s not like it’s an either/or skill; people who don’t subvocalise don’t necessarily don’t have the ability to subvocalise, it’s just that we choose when, what for, and why to utilise it.