Most of us imagine that we read one word at a time from left to right along the page. The situation is in fact a little more complex than that. The physiology of the eye is one factor that can and does limit how fast we read.
Our eyes have a lens which focuses light onto the retina. For text held at arms length, we only see about four or five letters in perfect focus. This represents light falling on the fovea at the centre of the retina. Light falling further away from this central point gets ever more blurry. However, our brains are capable of correcting this blurriness so that our whole visual field appears to be in focus.
Our eyes are capable of recognising images flashed onto a screen for a very short period of time. We can recognise four words flashed onto a screen for a mere 1/500 seconds. However, we can only process about ten separate images a second. This is why TV and film which plays at 24 frames a second seems continuous to us. It is faster than our eyes can process. Some animals such as birds of prey can process many more than 24 images per second. For them, TV and film would look like a sequence of still images.