The slow motion description of the bombs is used to demonstrate the full effect of Montag’s emotions.
Montag goes through a lot of change throughout the course of the novel. He goes from being a fireman who loves to burn books to a being a secret caretaker of them. When he sees the bombs exploding over his city, he realizes that his old life is gone for good.
Once the bomb-release was yanked it was over. Now, a full three seconds, all of the time in history, before the bombs struck, the enemy ships themselves were gone half around the visible world, like bullets in which a savage islander might not believe because they were invisible…. (Part 3)
The people have always lived with a false sense of security. They believed themselves to be safe, and did not think that the bombers would ever come to them. The ongoing war was little more than a footnote to them. It was never real. The slow-motion here captures that sense of reality slowly sinking in, and the knowledge that everyone died.