At my somewhat-below-fastest (which is still faster than my current normal, as I'm a lazy reader these days) my concentration, recollection, and enjoyment was the best -- plus I finished everything way faster. For anything serious (and I mean you seriously want to enjoy something or seriously want to grok something) you will have to slow down. But that doesn't mean you can't go faster than your current fastest and still get those other things out of reading that you want. It's all in the training. Don't train with William Faulkner.