It isn’t about how interested they are in something. If you have a text that is both written in a confusing way and also contains concepts that are difficult to understand, it becomes impossible to speedread effectively. I highly doubt you can read Kant’s Critique of Pure Reason at 100 WPM if you have never read it before. It is impossible to read something like that and understand it without frequent and lengthy pauses to think about what the philosopher is actually saying. My argument is that speed reading isn’t all that useful in day-to-day life because whenever you care about what you are reading, what matters is if you can understand the content inside and not the words.