It was a slow burn.

Meaning

The plot developed gradually, building suspense.

Examples

• John: Did you enjoy the movie?
• Alice: Yes, but <b>it was a slow burn</b>. The suspense built gradually.
• John: I prefer that to a fast-paced thriller sometimes.
• Alice: Me too. The slow reveal made <b>it was a slow burn</b> even more impactful.