Dear Downvoter,
I'm not sure whether it requires more concentration, but it may very well be that like you know, some people read well and it's nothing for them to read a book a day. And other people this is a big deal. I mean it's a struggle, and what's likely is going on is that most people never learn to read very well. And so, the amount of concentration they have to put into the mechanics is what's defeats them with the amount of material that has to be read.