Hawthorne has an incredible gift for descriptive passages, and some of his paragraphs about the Pyncheon garden make us feel as though we're burying our faces in roses as we read. But his love of description means that the plot development is not quick: Hawthorne draws out the suspense of The House of the Seven Gables almost frustratingly slowly. After all, the plot itself isn't the goal: Hawthorne is trying to paint a true psychological portrait of each of these characters, which requires a lot more time than, "A did this, then B happened."
Let's take, for example, this passage from a moment when Clifford, Hepzibah, Phoebe, and Mr. Holgrave are all having desert in the Pyncheon garden:
Meanwhile, Holgrave took some pains to establish an intercourse with Clifford, actuated, it might seem, entirely by an impulse of kindliness, in order that the present hour might be cheerfuller than most which the poor recluse had spent, or was destined yet to spend. Nevertheless, in the artist's deep, thoughtful, all-observant eyes, there was, now and then, an expression, not sinister, but questionable; as if he had some other interest in the scene than a stranger, a youthful and unconnected adventurer, might be supposed to have. (10.23)
Mr. Holgrave is spending a lot of time trying to draw Clifford into conversation. You might think he's doing this out of love for his fellow man, but you'd be wrong. "Now and then" a "questionable" expression suggests that he has "some other interest in the scene than a stranger." Hawthorne's use of the words "sinister" and "questionable" cast a dark tone on a scene that has, up until now, been quite pleasant and domestic.
Hawthorne is clearly foreshadowing something. He might as well be drawing asterisks and exclamation points around this observation that Holgrave's interest in Clifford is more than an average stranger's would be. By the end of the book, we find out that he is a descendant of Matthew Maule, but that's another eleven chapters away. This heavy foreshadowing makes everything that happens in the book feel weighty and fated – one might almost say gloomy.