I just came across this interesting article, which describes a study showing that dyslexics use almost 5 times more brain area when processing language (real vs. non-words and do the words rhyme or not). This is likely also true when they are reading, as reading is directly tied into language processing and this type of phonemic awareness skill.
No wonder your dyslexic students are so tired after reading a passage. No wonder they often read slower than their peers. No wonder they struggle to keep their attention when reading. And no wonder they have to work much harder just to keep up with their peers in school.
An Orton-Gillingham program works to build a stronger, more efficient reading pathway in the brain (starting with those auditory skills of rhyming, and using non-words to engage those auditory processing centers of the brain). We re-train the way a dyslexic student is processing sounds and reading, so they are not using so much of their brain when dealing with language and reading.