I thought that what OP meant is that if you can only run the simulation for a relatively simple CPU at, say, 1 MHz, you can run and debug firmware for a 30 MHz CPU effectively, even if it's a little annoyingly slow - your 1-second init routine takes 30 seconds, but that's OK. If you have a 1 GHz CPU, whose more complicated architecture can only be simulated at 500 kHz, then a simulation is really tedious. A 20-second boot IRL of, say, Android takes half a day on your simulated processor.

That's not to say there's not someone coming in this morning to step through the overnight simulation they ran, but it's a lot easier to work with when your simulation runs nearer real time.