I don't think the number of days it takes to read a book shows anything about being a fast or slow reader. Some people may read 4 hours a day, or 2 hours a day, or 1 to 1.5 hours a day (like me). If I am really into it, I will read every word. If I am not really into it, but not so turned off that I just forget it, I will tend to skim a lot more. I don't know what this means for my reading speed, but I consider myself to be generally a slow reader. When I was a teenager I took a speed reading course and could read very fast, but I found that while it was good for reading material I didn't like (like textbooks), it was not very enjoyable for my personal, "fun" reading. So I haven't speed read like that in many years - no, make that many decades.
Steve