Original Commentary
Brisbane and Kimberly aren't slow.... just really, really cautious.

Modern Commentary
Spoiler alert: She's sick.
But it's good that they're officially going out. It's easy to do what's safe and comfortable, which is why they're taking things slow. If things go bad, he loses his job and house. She loses her only friend. So while they both want to take things further, they certainly don't want to lose it all if things go wrong.- Terrence