How does Harper Lee generate tension in To Kill a Mockingbird in the mad dog scene?

First, Calpurnia calls Atticus, then runs up and down the neighborhood as far as is safe to warn the neighbors that there's a mad dog coming (and having the switchboard operator call everyone else). She even bangs on the Radley's door to warn them, although no one answers. Meanwhile, Tim Johnson--the mad dog--is slowly walking toward them. Atticus and the sheriff arrive and discuss what to do--go after him or wait. 

Scout--narrating--says, "I thought mad dogs foamed at the mouth, galloped, leaped and lunged at throats, and I thought they did it in August. Had Tim Johnson behaved thus, I would have been less frightened." She's a frightened child, watching a mad dog moseying toward her neighborhood while the adults--also clearly unnerved--try to work out what to do. 

Atticus seems to move slowly and speak softly from the moment Tim Johnson limps into view. Then Tate and Atticus argue about who will shoot the dog. Tate practically throws the rifle at Atticus and says, "I'd feel mighty comfortable if you did it now." Scout and Jem are shocked to their very core, of course, because they think their father is and always has been anti-gun. 

When Atticus walks into the middle of the street, Jem and Scout watch "in a fog": "He walked quickly, but I thought he moved like an underwater swimmer. Time had slowed to a nauseating crawl." We naturally witness frightening events as though they are in slow motion, because our "internal clocks," pumped with adrenaline, move so quickly that the outside world seems to move in slow motion. This is what's happening here. 

Atticus is in the street, shoves his glasses to his forehead, and they fall off and crack on the pavement. Lee then turns our attention to Tim Johnson, who has finally worked out where the street is and starts coming down it again, having turned at the Radley place: "He took two steps forward, then stopped and raised his head. We saw his body go rigid." This is the moment--do or die. 

Then all at once, Atticus yanks the lever, pulls the rifle to his shoulder, and fires. Tim Johnson leaps and flops over in the crumpled heap. Up to this moment, we--like Scout and Jem--know nothing of Atticus's history. We don't know for sure until this moment that he is a sharpshooter, so we--like Scout and Jem--are on pins and needles watching the scene unfold.