I do something similar, only keeping my lips shut and moving my tongue and throat as if I was speaking. I find it's an intermediate speed between conversation speed and purely reading with my eyes. I started doing it when I wasn't so good at English to give myself time to understand the text, as well as to practice the mechanics of English speech when I didn't have anyone, but I find keeping me at this pace gives me maximum comprehension. I have a friend who reads much faster than me and he quite often misses points in whatever he's reading. I think he got into that habit from literature, but it's disastrous when reading something more densely packed with information, like technical documentation.