It depends on the work, writing, my state of mind, etc.
As I learn more about a topic, bad writing gets easier to read (there's little in it), good writing gets harder (it keeps jogging other associations or revealing new sources). I have to actively assimilate that information. Increasingly, I track down interesting references as I'm reading, if possible.
I can read an easy novel (two of the most recent: The Martian and The Circle) in a few hours. It may take me months or years to work fully through a meaty book, though a week or so is possible if I'm focused. (I rarely am.)
This incidentally makes working with library loans almost impossible -- I don't have as much time with the material, and my reading style is quite active, marking up, starring, or making marginal notes. That's fine for my own copies of works, it's exceedingly disrepectful for shared texts.
(Though I still have fond memories reading through a uni library copy of a Jane Jacobs book in which an earlier reader had penciled "God bless you, Jane" to a particular passage. I agreed with the sentiment.)
For focusing: sometimes I just tell myself to focus on the material at hand, take quick notes if necessary (index cards), and plough through. When that gets too hard -- after an hour or two usually -- I figure it's time to stop and switch to something else.