Sometimes you really have no choice though, such as an investment banker faced with a mountain of research (literally thousands of pages) to parse through in a few days. Although then the subject might be pure skimming / info mining rather than reading proper.
This is exactly what I went through. I'd have way more to read than I knew was actually possible and so I'd scan for typically important, typically worth writing about information, but I wouldn't really get a complete picture. There are techniques to help you appear to be competent in what was written, even for complex work, but always when I've gone back at a more relaxed time to read it properly (say after everything had been finished lol), I've discovered a great deal I missed and could have talked about. Whereas, of course, there's some things with almost no content or thought put into them that whether read slow or fast you won't get anything out of them. That's what I meant to illustrate, that there is always value in reading slowly what it worth reading. You will get less out of it if you have to read it quickly.