Do you mean talking in your own voice in your head; you want to stop doing that? So as to no longer hear your thoughts as words but just think?

I think it is very difficult for anyone to do for more than a few minutes but with advanced levels in yoga / qi gong /etc. mindfulness it has to be possible, and for most normal people would be beneficial.
To properly "speed read" you have to stop hearing the words you are reading; just see the sentences on the page and perceive their meaning visually without the extra step of translating it into imagined sounds. Lots of people have taught themselves how to do that, give it a try first and then see if you can shut off your inner monologue the same way.