I decided to make this post after analyzing my own reading speed and the reasons why it is so fast... but I also wanted to give tips on how to increase your reading speed without hurting your enjoyment of VNs, manga, and books. At the same time I was doing this, I gradually came to realize that one of the big reasons why my favorite genre is so niche (chuunige) is simply because most people bump into the walls of complex text and give up.
Regardless of which language you are reading (Japanese, English, or any other language), the basics of reading are the ability to see and understand the writing, have the vocabulary necessary to understand the words, and an understanding of grammar advanced enough to comprehend how the words come together to create meaning. I know it sounds condescending for me to go back and name these basics, but it is necessary to do so in order to make my points.
When it comes to reading fiction, there is an aspect that comes into play that many don't take account of... the gap between the language used when writing and that which is used in verbalization. Most people won't use even a fifth of the terminology and styles that exist in the literary world to converse with others (at least, if their job doesn't require it). For this reason, becoming a fast reader necessitates that a person have a gut-level understanding of a large amount of word and style choices that simply don't get used when they are conversing with others. This is also the reason why classroom Japanese is inadequate for playing or translating visual novels, incidentally.
To be frank, the above reality is the main reason why reading for pleasure is an acquired taste for most, rather than a natural addiction. To me, a well-written scene in a VN is a sensual, almost erotic experience... but that is because decades of reading have made me into that kind of person.
To be frank, there is a limit to reading speeds based on intelligence and short-term memory. However, this limitation is far less of an issue than most think. It is quite possible for someone who is of less than average intelligence to be able to enjoy reading something as ridiculously complex as Dies Irae at a speed you might be surprised at, and it is quite possible for a person who is otherwise of high intelligence to have a turtle-like reading speed.
Basic methods to increase reading speed include deliberate expansion of vocabulary (memorizing lists of words and how they are used) and deliberately exposing yourself on a regular basis to writers with unique or unusual styles that are difficult to follow. Grammatical understanding needs to be gut-level or reading speed won't improve, as you'll be constantly stumbling over how the words come together.
Context is also important... essentially, to be a fast reader, one must be able to keep at least a decent grasp on what has gone before and be able to at least retain most of the details from the chapter you are currently reading. A bad habit many translators, both professional and fan, fall into is translating line by line. This is also a stumbling block when reading. If you are merely reading line by line while not keeping at least some track of what has been going on, you will be unable to grasp what is coming. This lack of understanding slows the reader, as they grow confused, then bored. One reason why I almost never play multiple VNs at once without dropping the previous ones outright is because retaining a firm grasp on the flow of events at my stopping points for those games causes an intellectual and emotional disconnect that makes it hard to resurrect my interest if I try to go back to them.
In the end, what was this post about? Essentially, I was saying 'refine your basics, and your reading speed will improve'. I'm not going to go into more advanced techniques such as being able to 'flash-read' paragraphs and lines, because I generally don't use those techniques when playing normal VNs (the slow death of the NVL format has ensured it only has limited usefulness when reading VNs).
A note about the difference in enjoyment: The pleasure gained from reading varies in nature as you get faster. One reason I love VNs that are heavy on complex narration is that such VNs rarely leave me feeling that I wasted my time reading them, whereas ones with little narration and most if it simple tend to leave me feel like I wasted my time. In my observations of others, including some friends I introduced to VNs who have significantly slower speeds than myself, I saw that they tended to be more able to enjoy both better than myself. However, since large-scale VNs seemed like monumental tasks to them, they often don't even try them. In this sense, reading speed determines what some people read in the first place, thus narrowing their options and experiences. I hear stories about people taking months to finish a single VN, and I honestly can't comprehend that, given that even the longest VN only takes me about thirty to forty hours (incidentally, only a half dozen VNs have reached this threshold in my personal experience)... which is about the same amount of time full completion of an average-scale jrpg takes, lol. I honestly don't have the experience to gauge how to enjoy a VN over the course of a month or more of time... and I'd like some input on just how that experience feels, since I don't have any references in my experience that might enable me to understand it. Even my friends were book-addicts before I introduced them to English-language VNs, so they were still faster than the average...
Edit: What caused me to decide to post this was the simple fact that I have, on a number of occasions, been asked for advice on how to improve reading speed. In the process of trying to articulate how to improve reading speed based on my experiences, I decided to focus on visual novel reading speed, because the explanations for some of the techniques I've picked up over the years would require me to rediscover how-to books I read back in my teens when I was trying to reduce the amount of time I had to spend studying by finding ways to read faster. I had to laugh when I realized that all I could do was explain why the basics were important, because the most important element for reading VNs turned out to be accepting that the vocabulary you pick up reading the 'classics' in your years of public education is not nearly enough. Vocab and an understanding of how it fit into grammar are so vital to reading quickly, because you don't have to stop to think about what a sentence means if there is no part of it you don't already understand.