So, as far as I know the cycles engine does ray tracing until it hits a noise threshold, then does ai denoising for the final cleanup. You can see where the more visually complex parts of your render are, because it will take a lot longer to render to a less noisy state. I don’t know specifics of how it works under the hood, but given how complex parts of your image take longer to tender to an acceptable threshold than simpler parts it seems obvious to me that render time scales with complexity.