The luminosity of a black hole is not determined by its mass, but by its accretion rate. A black hole becomes a quasar or an active galactic nucleus when it is actively consuming large amounts of gas and dust. This falling material heats up due to friction and gravity, emitting massive amounts of radiation across the electromagnetic spectrum.
When a supermassive black hole is described as calm or stable, it means it is currently in a quiescent state. This occurs when there is an insufficient supply of nearby matter to fuel the accretion process. Without a steady stream of gas to fall into the event horizon, there is no significant friction or heat generation, meaning the black hole remains relatively dark and quiet.
In short, mass provides the potential for great energy, but matter provides the fuel. A 9 billion solar mass black hole can remain dormant for billions of years if its surrounding environment is depleted of enough fuel to trigger the violent activity seen in active quasars.
