6. Reading is Unnatural

Speaking and understanding spoken language is a natural human ability, but reading is not. However, this ability decreases after the early childhood. The learning and processing are handled by different brain areas from those that handled it in early childhood.

Using verbal language existed thousands or millions years of human existence, while written language was just became common in only a couple hundreds years ago.

Learning to read = training our visual system. 
Lines, contours, and shapes are basic features that human brain recognize innately.
Basic features combine to form patterns. Characters, letters, numeric digits, and other standard symbols.
Patterns of characters form morphemes, such as, -ed, -ing, basic words.
Words combine to form bigger patterns, phrased, expressions, sentences.
Sentences combine to form paragraphs
While brain does that, our eye movement is trained to scan text efficiently. Saccade.

Feature-driven, Bottom-up reading
Natual ability to recognize basic features, lines, contours.

Context-driven, Top-down reading
Learned ability to recognize patterns, words, phrases.
Once taught, human brains favor this over feature-driven.

_note - foreigner, improperly trained adult can't perform context-drive reading hence can't read fast.

Factors disrupt context-driven reading
Uncommon or unfamiliar vocabulary
Difficult scripts and typefaces
Small font sizes, styled text, all capitalized paragraph
Poor contrast and background that blocks scanning
Repeated similar patterns (forces feature-driven reading to confirm)
Centered paragraph without aligning (forces eye movement)