Yes agreed it looks like it would take ages for the planet killer to destroy a planet at the rate we saw in the episode. Would it actually be possible ? Wouldn't gravity stop it? What about oceans?While the planet killer is huge compared to a starship it is actually a tiny thing relative to a planet. Cutting up a planet could actually be a time consuming affair let alone digesting the entire planet. It might only require taking bite sized chunks out of a planet, which could still be quite devastating. It could also more easily consume asteroids, comets and small moons for fuel.
Consuming planetary matter is likely just a convenient side benefit to it’s actual purpose as a weapon of terror—a near unstoppable device to destroy inhabited planets or powerful defending/attacking spacecraft.
I'm picturing when it is dealing with a entire planet rather than a starship it sort of torpedoes or anti-matter blows up the planet - using its cutting beam for fine work such as dealing with starships and to get the fuel out of the chunks of planet