It would be possible to filter the exported objects to just the ones into the camera field ?
During complex (lot of objects) animation the big part of the job is exporting not rendering.
1. I don't know if there are function in Blender that returns the objects viewed.
2. You are missing the lighting contribution due to filtered objects (could be small or big depending on the scene).
In case 1... not sure you can really do something.
In case 2...well a button can enable/disable this filters.