Speed reading is not just about reading faster; it's about learning to use much more of the extraordinary powers of the Mind.
When you read, are you aware of an inner voice that follows the words as your eyes move across the page or the computer screen? This inner voice is called 'subvocalization'. You probably experience it as a slight movement in the tongue or throat region. As long as you subvocalize, you limit your reading to the speed of normal speech, to about 300 w.p.m.
The Mind is capable of thinking much faster than that. So when you subvocalize, you're literally holding back your mind. Try this exercise:
As you read, count to yourself, silently, from one to ten. Or, repeat the sound 'Eee', 'Eee', 'Eee'. It will be impossible to do this at the same time as subvocalizing, so this is an excellent way of breaking the habit of subvocalization.
As you do this exercise, you'll become aware that you're no longer processing the words in the tongue/throat region but in an area called 'thought stream' that you experience in the top of your head.
Thought stream moves much faster than subvocalization. And that's why people who subvocalize often have comprehension problems.
There's a mismatch between reading speed and thinking speed. The Mind is constantly racing ahead of the inner voice and so it gets bored. You experience this as an inability to hold your attention on what you're reading. You have to back-skip words, or read the same line twice.
As your reading speed catches up with your thinking speed, reading becomes much less tiring and your comprehension improves.
Once you've got a feeling for reading in 'thought stream', the next thing to do is speed up your eye movements. This will also help break the habit of sub-vocalization, since your eyes will be moving faster than you can possibly subvocalize.
Your eyes move across the written page in a series of quick jumps. Between each jump there's a stop lasting a fraction of second, called a 'fixation'. The fixation is when the eye actually takes in the written word.
The untrained eye takes about a quarter of a second at each fixation, and takes in 2 or 3 words per fixation.
By speeding up you eye movements, you'll learn to make fewer fixations per line and take in more words per fixation.