You're not expected to help every single person you encounter.
Cindy held the door open for the person behind her.
Cindy wants to be friendly.
Cindy holds the door open for the next person, then walks in.
Cindy holds the door open for the next person, and the next, and the next...
Cindy gets on to her next task quickly and efficiently.
It's an hour later before the front door is empty enough for Cindy to leave.