I think there is some truth in the advice to avoid sub-vocalizing. It seems to improve my reading speed, but does take an effort. The other day I was reading something that included a word I have some trouble pronouncing (sorry, don't remember what it was now). I noticed I was pausing over that word every time I encountered it, trying to solve the puzzle of how to pronounce it. When I made an effort to stop that, I sped up my reading quite a bit.