I think speed reading is most useful when you are reading some text where only parts are relevant or new to you. By quickly reading those, you can focus on the important pieces and spend more time to think about them.
Interestingly, the faster I read, the less I'm able to "filter" the information I read about, it seems almost as if my brain absorbs it directly. That means that I'd never speed read content that might contain bad memes or false information: political propaganda, religious texts, etc.
Does anyone know a good test for reading speed (including comprehension)?