Ehri distinguishes 4 ways to read words:
“The first three ways help us read unfamiliar words. The fourth way explains how we read words we have read before. One way is by decoding, also called phonological recoding. We can either sound out and blend graphemes into phonemes, or we can work with larger chunks of letters to blend syllabic units into recognizable words. Another way is by analogizing. This involves using words we already know to read new words – for example, using the known word, bottle, to read throttle. Another way is by prediction. This involves using context and letter clues to guess unfamiliar words. The fourth way of reading words is by memory or sight. This applies to words we have read before. We can just look at the words and our brain recognizes them.”  [boldface mine]
[Note: If you’re unfamiliar with the terms “decode,” “blend,” “segment,” “grapheme,” or “phoneme,” you can find easy-to-understand definitions here. Understanding these terms is a necessity for reading this blog.]
Since the first 3 strategies for reading (above) all involve conscious effort and time, using any of them will impede reading comprehension. Reading by sight, however, requires no conscious effort – all the brain’s resources can be directed toward comprehending the text.
“Given that there are multiple ways to read words, consider which way makes text reading most efficient. If readers know words by sight and can recognize them automatically as they read text, then word reading operates unconsciously. In contrast, each of the other ways of reading words requires conscious attention. If readers attempt to decode words, to analogize, or to predict words, their attention is shifted from the text to the word itself to identify it, and this disrupts comprehension, at least momentarily. It is clear that being able to read words automatically from memory is the most efficient, unobtrusive way to read words in text. Hence, building a sight vocabulary is essential for achieving text-reading skill.” 
If you’re a skilled reader, you’ll likely read every word in this blog effortlessly – by sight. A mere glimpse of each word will immediately link to that word’s pronunciation and meaning. The brain’s ability to do this is astonishing. How does it happen?
A traditional view of how sight words are created holds that beginners memorize some type of association between a visual characteristic of the word (perhaps its overall shape) and its meaning. The pronunciation of the word is activated only after the meaning of the word has been retrieved. Ehri calls this notion “incorrect.”