It seems to me that since the Opah has to move its fins really, really fast to keep the blood circulating through its body and keep the fish warm, that it probably expends a lot of energy while doing this; just like we would do if we were in a cold place and trying to run or jump to keep warm. It is probably a very good thing that the Opah is fast enough to catch the unsuspecting and slower fish that it eats; because I would guess that it has a much higher caloric requirement than the semi-dormant fish have.
You would think that after all these years, and all of our modern day equipment; we would not still be finding these creatures that we did not even know existed, and even some that were thought to have become extinct millions of years ago have been discovered living in the depths of the ocean.
Even Panda bears were though to be a myth until well into the 1800's; so who knows what creatures we have yet to discover are real that we also thought were a legend and not truly existing.