As important as I've learned reading is to writing, there are some months when I find it a chore, some times when opening a book and reading a sentence are onerous tasks. Where one book will take five days, another may take five weeks. And often enough it's not a matter of the writing; it's a matter of the attention I can bring to the task.
I can be easily distracted by TV or the need to look something up on the Net. These and many minutiae can pull me away from reading. Ideally, I need to be in a spot with good lighting, quiet enough so I can hear the rhythm of the characters' speech or, in non-fiction, the author's reasoning. I suppose I could skim, but that to me shortchanges the author. I would like to think authors consider their words carefully, and deserve to be read carefully. The equivalent of skimming in conversation is folding your arms and tapping your foot, waiting for the speaker to come to a point. Or worse, simply waiting for your turn to speak.
This is not to say I remember every word of every book I've read. Some wording really is meant to get from Point A to Point B while readers are caught up in the pacing of the overall language. My point is I can't recognize something as often subtle as pacing unless I'm focused.
Finally, there is something to be said for getting out and participating in life, not trying to get away from, say, the train ride to work by reading. Then again, the ordinary is often what grounds fiction, what makes it seem as that, yes, it could happen. So I like to focus on the ordinary.
When is my best reading time? Early morning, after nature calls, but before it's reasonable to start the day's work.