There's another aspect to the question - how engaging or how people feel about speed reading when it comes to being immersed in a story (assuming you read a novel with that technique, for example). Most of the Speed Reading demos out there focus on reading short paragraphs and that's it. Even if comprehension is not a problem, how fast and how feelings develop in your brain as you read may not be comparable between traditional reading and speed reading. That can make a huge difference on how much you enjoy the piece of text you are reading, not matter if you can still get 100% of it.