Slowly.
No. Natural selection works in all populations. However, new alleles spread more slowly in large populations; the large size has a stabilizing effect. So one should expect large populations to change more slowly than smaller populations.
Slowly press on the accelerator to go more quickly. Slowly press down on the brake pedal to go more slowly. These two operations can safely change a car's velocity.
Both, depending on the weather, big storms cause the fastest changes.
SLOWLY
slowly
slowly
slowly
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slowly
The opposite of quickly is... slowly.
Slowly by cellular respiration, quickly by combustion (burning).