low vibrational frequency allows the devils to be in hell without actually suffering much

It is only when the devils try to come upwards and to better living conditions that the suffering begins. The laws of spirit would force them to face their rotten interior and cause pain that correspond to ones life and nature. And it would take astronomical amount of time to generate a good life if a person is in hell.

When they are in hell only when they try to break certain laws will they be punished accordingly by other devils. The devil has destroyed all his interior so no real interior pain is for him possible even he is living in horrible conditions.

The devils are thus free to torture all sinners during earth life or in spirit world during purification without themselves being in much pain. They actually enjoy interiorly this hellish fire and inflicting pain.

Hell is both trap and sanctuary to devils and they live there by their own free will in quite a low state of human existence. It is the only place they could be. This is the reason such places are allowed to exist.