Subvocalizing limits you to about 5000 words per minute—which is still ten times faster than a “fast” reader. In the speed reading course it was a wall that some could not get through. It was explained to us like this—when you see a stop sign, do you have to say “stop” to yourself before understanding it’s message? No your brain understands it without “hearing” it. I think subvocalizing is a remnant of how we first learned how to read—reading out loud. When we were taught to “read to ourselves” we just thought the words. If you hold your throat while you read, you can feel the muscles working away.
You’re absolutely right about subvocalization being a relic of voiced reading (reading aloud). I can overcome it if I really work for speed, but it makes a labor of a love. So unless I’m just trying to blaze through something ridiculous like “Ulysses,” I subvocalize and devil take it!