The main obstacle when learning how to speed-read is subvocalisation.
Subvocalisation is where you sort of 'say' the words in your head as you read them, so you can't read any faster than you speak.
Breaking this 'speech barrier' will allow you to read much, much faster.
However, bear in mind that reading faster won't necessarily allow you to learn faster - there is usually some level of trade-off between comprehension and speed.
Personally, when I read textbooks and such, I'm quite slow because I think about what I'm reading as I'm reading it... it's got nothing to do with how fast I can read, if that makes sense.
A good friend of mine from high school is an amazingly fast reader... she read Harry Potter and the Half Blood Prince cover-to-cover the day it was released.
Edit: I just asked her... she said it took her 4 hours - she wanted to read it before anyone could ruin the ending for her. She's easily the fastest reader I know.
http://en.wikipedia.org/wiki/Speed_readinghttp://www.wikihow.com/Learn-Speed-Reading