In most, if not all, rendering images (taken at medium to short focus distance), the Apo-Summicron is shot at mostly a third stop faster shutter speed than the Apo-Ultron, while having the same exposure in the central area.
Given that, as well as the size of the rear lens element, I wonder whether the Apo-Ultron is more like an f/2.3 lens, at least in practical use. I highly doubt glass and coatings are the reason. I speculate that the quite strong optical vignetting which is visiblea little away from the center of the image is the reason for the difference we see in shutter speeds.
