All hypothesis, dredged up out of cold storage from over a decade ago, and I may not have understood it then, let alone these fragments left to me now.
More mitochondria, fixed resource inflow, faster resource burn, more toxins produced in the same time, shorter run time (due both to cleanup being impaired/swamped as respiration is impaired, and resources being used faster than they can be replaced).
So being "fitter" may equate to being sicker, stronger, but with less power, more severe PEM.
Only being one person, with a slightly iffy memory, I am not in much of a position to comment as to any hypothetical validity of this hypothesis.
ETA - it looks like I am assuming that the inflow and egress of stuff through the cellular membrane, and possibly internal membranes, is impaired - this would seem to be correct, I do think this, but I am currently unaware of any actual evidence for this, science wise, so quite why I think this would appear to be unclear.