How does the careful attention to detail affect the narrative pace of part 1, "The Tallis Estate"? Does it serve any purpose? How do things begin to "speed up" as part 1 comes to an end?

In the first section of Atonement (Ian McEwan), we see Briony Tallis as an inquisitive young girl who loves to write. One of the very first scenes involves the reader listening to Briony convincing her cousins to perform the play that she has written. The narrative pace is set by Briony, as we later discover that Briony herself is writing the novel as an old woman to atone for her past mistakes.
Part of the reason the novel moves slowly through part one is to properly set up the events that lead to Robbie's imprisonment. The reader learns that Briony comes from a wealthy, educated family and that she has a crush on Robbie, who, in return, shows interest in her older sister, Cecilia. Because the entire novel revolves around Briony's accusation of Robbie at the end of part one, it is necessary that the reader understand the complications of that accusation. We learn that Briony loves to make up stories but that her innocent scheming causes real damage.
As the reader understands Briony's world, the novel can begin to pick up speed. Things happen quickly: the search for the twins, Briony stumbling upon her cousin Lola being raped, Briony's accusation of Robbie, and Robbie being taken away by the police. Part two leaps forward several years to World War II, where Robbie is now a solider reminiscing about Cecilia. Without such a strong foundation to set the novel up, Robbie's memories might not carry as much meaning.
Another reason that the narrative pace in part one pays such close attention to detail could relate back to Briony's character development. It's clear that Briony observes everything around her, and by having her do so in the book, McEwan shows the reader how Briony acts rather than simply telling us that she is observant.