Yes, that tedious stretching does indeed come from the book, rather than being inherently the fault of the film - and what a pity David Yates wasn't brave enough to change it! It's annoying in that book five in particular came across as really rushed in the film version, and now the one book that could actually rather do with some editing down to make a pacier film is the one that gets a double-length feature allocated to it. Gah!