The theoretical limit of how efficiently you can turn energy into useful work depends on how cold your energy sink is. The coldest energy sink in an ever-expand

The theoretical limit of how efficiently you can turn energy into useful work depends on how cold your energy sink is. The coldest energy sink in an ever-expanding universe gets colder over time as everything spreads out and the fixed amount of energy occupies more and more space. So the hypothetical optimal end-state of an extremely old civilization is hibernating a bunch of very stable mass until the extreme far future, at which point they can drip-feed it into a black hole to do some extremely efficient computation.