In Permutation City, there's a fantastic moment when they realise that they can speed the simulation up, but they can't slow it down below a certain point, and they work out that the reason for this is because they are the ones being simulated by what they think of as their simulation. When they think they're speeding it up, they're actually just slowing down its simulation of them, and there's no lower limit to that, but there is an upper limit to how fast it can simulate them.