[tsimionescu]

Subvocalizing is much faster than speaking. In general, subvocalizing refers to pronouncing the words in your head, which can be picked up by observing neural activity in the vocal chords.

Even if your vocal chords were doing the full range of motion (they are not), when speaking, you also have to open and close your mouth, move your tongue, take in breath and expel it etc. It's much more than simply some activity in your vocal chords.