Leveling is not so crucial as ensuring that the machine is evenly supported on all four corners. There's no way to 100% avoid all vibration.

Many washer tub assemblies oscillate more at a specific speed range during spin acceleration, then settle down when that speed is passed as acceleration continues. Some machines have a sensor that detects excess oscillation and moderates the speed to keep it either or above or below that point ... which takes a few seconds to accomplish ... 30 seconds is not unreasonable.

Some also have a gyroscopic balance ring integral to the drum, containing water or other fluid and some also contain free-floating ball bearings that position opposite the heavier part of the spinning load to offset it.