It’s impossible to know if the content itself is worth the extra time and effort. Opportunity Cost is especially high in fiction. I agree that LotR should be on that list though.

When you start thinking about books in terms of opportunity cost, you’re the kind who needs to slow down the most.

I partially agree, but it depends on your objective function. If you’re optimising for the enjoyment of the experience of reading, you don’t need to know if the content is “worth it”; just read it slowly and if you’re not enjoying it, stop.