Of course, it assumes people aren't unfocused and dull-witted. I can easily read several pages, meditating on how fast I can read and then have to go back. Or I'll read the same sentence 5-10 times before the comprehension section of my brain starts firing and I can proceed.
Part of this comes from lack of practice, mental distractions ("why am I reading and not doing what I need to do?"), and personal exigencies, Or it may just be my reading choices.
Joseph Conrad's Lord Jim is not a focused book. It has two-three-sometimes-four layers of narration and skips and hops back as often as it ruminates on romanticism. And I catch myself distracted by vocabulary. It's vexing.