Performance implications of stereo SBS vs Over / Under in panoramic shader videos

Are there any performance implications of videos with SBS aspect ratios vs Over / Under when using the panoramic shader? Stereo SBS videos have a 4:1 aspect ratio, which makes it seem possible that they're less performant than a 1:1 Over / Under video source, but I don't understand enough of what's going on under the hood to know for sure.