The part of the universe that is empty also provides nothing you can use for a computer to run a simulation on, i.e. you don't have to simulate it but it doesn'

The part of the universe that is empty also provides nothing you can use for a computer to run a simulation on, i.e. you don't have to simulate it but it doesn't help running the simulation either. And given that we probe the entire visible universe looking at gravitational waves from across the galaxy, spectral lines in the light of galaxies at the other end of the universe, and the decay products of Higgs particles, you still need a quite good simulation or good heuristic approximations for a lot of stuff in order to not produce inconsistencies. Or you have to limit what your simulated humans can do and keep them actively away from discovering your speed optimizations. Faking parts of something as complex as our universe without it becoming obvious would probably not be a small feat.