I found training on not backtracking and understanding eye movements allowed me to understand better ...
The eye has two types of movement: the one you use to move from one object to another; and then the one that follows an object. He recommends the latter when speed reading, using a pen or pointy thing as an aid, your eyes following the tip as you sweep it across the page. You can move it from side to side, or down the middle, or zig-zagging, etc.
You accompany this with a metronome, to move the pen in regular movements.
I think in time the idea is that you can abandon the pen and metronome once you've got used to reading this way.
1: http://www.beelinereader.com/individual
Note: I am the founder
In general, I’d say that speed reading is handy when you’re hunting for relevant information, and slow reading could be handy once you’re reasonably sure that you’ve found it. But up until that point, slow reading would be a waste of time (I.e. spending too much time on irrelevant information).