You can train yourself to speed read (and shut down the subvocalization that gets in the way, at least temporarily) so I don't think it's totally dependent on not having an inner voice at all, though that might help. It's interesting. (To train: use a metronome and your finger. Scan each line with your finger on each beat, have your eyes follow your finger and try to understand what you're reading by sight. Eventually you'll get to a BPM faster than even your auctioneer speed so your subvocalization just won't be able to keep up with your finger, and so to understand you'll force yourself to stop subvocalizing. Increment the metronome. Occasionally do stretch goals where you increment by a large amount at the cost of comprehension, then go back down to a speed still above where you originally started, and that becomes your new base. Eventually you can wean yourself off both but you may still need one or both every now and again to power through something at full speed and full concentration. At some point you may start processing multiple lines at once.)