Put another way, the problem with speed-reading claims is that speed-reading is really just another way of saying “skimming.” You can flash as many words as you like in front of your eyes, and though you may be able to understand each word on its own, they won’t mean much as a collective whole. Language processing just doesn’t work that way. 

Here's the biological reason why:

There is only a small area in the retina (called the fovea) for which our visual acuity is very high. Our eyes are seriously limited in their precision outside of that. This means that we can take in only a word or so at each glance, as well as a little bit about the words on either side. In fact, since the 1960s, experiments have repeatedly confirmed that when people “speed read,” they simply do not comprehend the parts of the text that their eyes skip over.