When is praise not a compliment?

One of the things I have noticed on researching a book through the Amazon review system before I buy it is that so often reviewers state that they were able to finish the book in two hours, three hours, four hours. Do they honestly think that is a compliment to the author? —J. Lesley, review of Persuasion by Jane Austen.

Such a compliment is like praising a meal for how quickly it can be devoured.