Saying words in your head can sometimes be helpful.  For example, when you are reading material that has technical terminology or vocabulary that you are not familiar. In situations like this, saying words in your head, or even out loud, can be a useful way to improve and expand your vocabulary.  Here’s another way sub-vocalization can be useful. If you have to memorize something word for
word, sub-vocalizing the words or saying the words out loud would be helpful. How do you think actors and actresses remember their lines? Reading out loud can help you memorize something word for word, but when you normally read, you very rarely need to know something word for word.  Most of the time you are reading to extraction information, ideas and details.  To boost your reading speed, you need to minimize sub-vocalization by saying only a few words per line.  If you say every word, you’ll be limited to your talking speed.
How do you know if you are changing this habit? If you start reading over 300 words per minute, you are probably not saying every word in your head (because you can’t talk that fast). If you are going over 400 words a minute you are definitely making progress and probably just saying some of the words in your head.

Shared from Iris Reading