As others have said, our reference of time comes from our own universe's rules.
Ergo if rendering 1 second of out time took 10 years of their time, we wouldn't measure 10 years, we'd measure 1 second, so we'd have no way of knowing.

Simulation theory is, at least for now, unfalsifiable. It's always a nonzero possibility because by the nature of it there's always a counterargument for any evidence against it, just like how most religions operate.