As children we are taught to read, first by vocalizing letters of the alphabet then vocalizing words. This was necessary then. And now vocalizing works against you. This is because even as you vocalize the words silently you are using your vocal cords. Your vocal cords move at a snails pace. Your eyes and mind are so much faster. There are techniques that teach you to read without sub-vocalizing.
One reason that you talk out loud when image streaming is to outpace your conscious editor.
