I think the main issue here is the QgsVectorLayer::extent() method. It is only getting the extents from the provider if there is a edit buffer.

To me it should just be:

• Get provider extents
• If there is a edit buffer with changed/added features, add those to the extent.

At the moment it's a if based on if there is a edit buffer or not. If there is no edit buffer it loops all the features, which is not something it should do.