My ability to read fast seems to come from two sources, and it appears to be natural.
First is energy, if I have a sober and sharp head, I can read fast naturally. A sober and sharp head comes from the replenishing of the physical body, thus it’s physical energy.
Second is the technique, it seems that when fast reading appears naturally, I skim through the paragraphs that are not so important and can be digested or retold in a few words/sentences. Basically fillers. It’s more like I can sense what’s important in a chapter and what is not. I would highlight the important parts of a chapter automatically when I come across them, and the unimportant/filler parts would be skimmed through. Not sure how it works but it seems like when you are going through fillers, it’s like 1-1-1-1-A-1-1-1, usually books tell the important information this way. A is the important and 1 is a filler.