It's a bit of a slow burner.

Meaning

The story or plot develops gradually, building suspense or interest over time.

Examples

• John: Did you enjoy the movie?
• Alice: It was interesting, but <b>it's a bit of a slow burner</b>. I had to really pay attention.
• John: I agree. The plot didn't really pick up until the second half. But the ending made it worthwhile.
• Alice: Yes, exactly! At first, I wasn't sure, but it all paid off. It really is <b>it's a bit of a slow burner</b>.