Individuals can be Verbal or Non-Verbal Thinkers
Verbal thinking is where the speed of speech matches the speed of thought; thinking with the sounds of words through an internal dialogue.
Non-Verbal thinking is much faster, outpacing the speed of thought. It is thinking in pictures or using a kinaesthetic approach. Most importantly, this way of thinking is evolutionary - the picture or thought grows with each additional word.
Most people think using both verbal and non-verbal methods, specialising in one. A dyslexic person is inclined to think non-verbally - they will therefore find the verbal approaches of traditional schooling a challenge.