Assuming that this happens only when the user is actively using the app, I'd guess that the display would consume dramatically more energy, making the camera's usage of the battery negligible, also compared to the benefit of then having a very responsive camera.
In any case, I do think that providing visual feedback to the user is very important. Like a small picture-in-picture-overlay.
In any case, I do think that providing visual feedback to the user is very important. Like a small picture-in-picture-overlay.