Artur, here's a great article about training yourself to read faster:
http://www.mindtools.com/speedrd.html
If you practice trying to break bad reading habits such as sounding out words (subvocalization) as you are now doing and make a conscious effort to practice reading faster, you should improve your reading speed very quickly.
I trained myself to speed read many years ago by reading newspaper articles as fast as I could and trying to remember what I had read. At first, I didn't remember much of what I was reading but I improved daily until I became a very fast reader. It's a skill that has served me very well over the years.