Some reason people read slowly is that they don't have confidence in themselves .They're not convinced that they understand what they're reading,so they read apassage several times,trying to make sure they haven't missed anything .But just like people who check that the front door is locked 15 times before leaving for vacation and still lie awake at night wondering if they locked the door ,reading and rereading apassage doesn't give you confidence that you understand the text .You get confidence from proving that you understand it.