I'm doing R&D.
The large image is needed for rendering because, as an environment decal wrapped around a sphere, a small portion of it will fill the camera view at any time and needs to be of sufficient res to fill that camera view. Since the camera will pan more than just that small portion is needed to be on the environment sphere.
However... my question is: if it can be shown clearly in real-time before any on-screen render is done, why not after?
Here is the before and after: