That depends on what I'm reading.

I sometimes read math papers - these are hard to read and I don't understand much, but I'm willing to deal with the chore because it expands my thinking long term to do so. People should read works like this that are at the edge of what they can understand. Even if there isn't a test, it will sometimes help with life (there is some evidence this delays mental decline and thus results in a longer life span - I'm not sure if it is true but an additional point)

When I'm reading "dragons vs rouges" or some such fiction I'm looking for fun so if the book isn't fun I drop it fast. I probably wouldn't get far with "Girl with the Dragon Tattoo" if what you say is true - there are more fun books I would enjoy than I have time to read as it is, so if fun is the goal I'm going to drop anything that isn't proven fun quick.