I've been in ketosis for a couple of years and I have noticed similar things after a longer ride at higher intensity. I just can't seem to get to an effort that makes me suck wind. I've never considered myself as having bonked, since I can usually go on for quite a while, I just can't bring the effort up. In fact, my heart rate usually stays pretty low unless I'm climbing a hill. Hill climbs will bring the heart rate right up, but no heavy breathing.