It would be trivial for an SI to run a grainy simulation that was only computed out in greater detail when high-level variables of interest depended on it. Most sophisticated human simulations already try to work like this, e.g. particle filters for robotics or the Metropolis transport algorithm for ray-tracing works like this. No superintelligence would even be required, but in this case it is quite probable on priors as well, and if you were inside a superintelligent version you would never, ever notice the difference.

It's clear that we're not living in a set of physical laws designed for cheapest computation of intelligent beings, i.e., we are inside an apparent physics (real or simulated) that was chosen on other grounds than making intelligent beings cheap to simulate (if physics is real, then this follows immediately). But we could still, quite easily, be cheap simulations within a fixed choice of physics. E.g., the simulators grew up in a quantum relativistic universe, and now they're much more cheaply simulating other beings within an apparently quantum relativistic universe, using sophisticated approximations that change the level of detail when high-level variables depend on it (so you see the right results in particle accelerators) and use cached statistical outcomes for proteins folding instead of recomputing the underlying quantum potential energy surface every time, or even for whole cells when the cells are mostly behaving as a statistical aggregate, etc. This isn't a conspiracy theory, it's a mildly-more-sophisticated version of what sophisticated simulation algorithms try to do right now - expend computational power where it's most informative.