Perhaps the "real" civilization could run the simulation slower than real-time. For instance, while a whole hour elapses in the real world, only a half hour might elapse in the simulation. To the person observing the simulation, everyone inside the simulation appears to be going slowly, while time flows normally for the people inside the simulation.
I think that this would reduce the computational intensity by a factor of 2.
This brings up an interesting question (or possibly interesting): how would one observe the simulation? Would we not be able to detect evidence of the observation?
My almost entirely uninformed idea was that observation of the universe by something that is external to the universe might somehow interfere with the conservation of energy or in some other way produce phenomena that contradict the laws of physics.
I think that this would reduce the computational intensity by a factor of 2.