I don't know about the parent, but I have tried this - repeatedly, in fact, including taking a number of reading comprehension exams each way, just to see the effects. Something that frequently happens is that I'll mix up side characters. I can only juggle a few characters at a time when speed-reading. I really made a mess of The Hobbit, back in the day, completely mixing up several of the dwarves. And I miss a number of nuances of language - a clever bit of wordplay might pass me entirely. Really, it comes down to the author's style - someone like Stephen Baxter might be on the harder side of sci-fi, but I'm able to chew through his books in hours because there are generally few extra characters. Everything just kinda... proceeds linearly. There's little to track other than what's immediately available. On the other hand, your typical high fantasy fare has to be chewed over the course of a few days if I'm to follow what's going on.