When you simulate something with increasing complexity you don't generally run said simulation at a fixed time interval that you then scale hardware to meet. At

When you simulate something with increasing complexity you don't generally run said simulation at a fixed time interval that you then scale hardware to meet. At the start the processing required would be minimum and thus the simulation would progress rapidly. With increased complexity the computation time per update increases and the general progression of the simulation slows down. At worst we would just approach complexity rivaling the parent realities and our updates would take substantial lengths of time in their universe. Whereas the first billion years of simulation might have taken seconds, it might become the case that one of our seconds can take years of computation given enough complexity. But for us we wouldn't notice either way - our perception of reality is bound to our experience of time.