What does attachment_fu do when saving?

Hi,

I have a model with attachment_fu.
The model saves images but I didn't put validates_as_attachment.

I understand that it takes long time to create an object as it has to
process images.
But I don't understand why it takes long time to update the object (not
changing file but only modify other non-attachment_fu attributes).
If I do object.save(false), it doesn't take long time.

So my question is...
What's the difference between object.save and object.save(false) when
the object is attachment_fu model and there's no validation?

Thanks.

Sam