Interesting topic. I'm at the point where I can tell when something I'm reading is lagging, but figuring out why is still tricky. My sister will only read fast paced books. I'm a bit more forgiving. I've read some books that were slow movers that I was fine with. There are others that just dragged on forever. I think genre plays a part (romance and fantasies handle slower paces better than mysteries or thrillers) but...I don't know. I think it depends on where an author decides to flesh things out and where to scale down. Maybe it's an experience thing, because some authors do this well and others just drag on forever. It’s an interesting topic to mull over.