It will fall fast if it is heavy and slow if it is light because the gravitional force is pulling it down to earth
A common answer to this riddle is "an investment." Investments often appreciate in value slowly over time, but when they decline, they can lose value rapidly. Another interpretation could be "a balloon," which rises gradually but can fall quickly if it pops.
Faster than on Earth? The reason it falls slowly on Earth is because of air resistance. You can also make it fall quickly on Earth if you make it fall within a vacuum chamber.
slowly
slowly
Slowly.
slowly
The opposite of quickly is... slowly.
Slowly by cellular respiration, quickly by combustion (burning).
slowly
slowly
slowly
Slowly