Verse 53:  They challenge you ‘O Prophet’ to hasten the punishment. Had it not been for a time already set, the punishment would have certainly come to them ‘at once.’ But it will definitely take them by surprise when they least expect it.
However, there are exceptions to this rule. Prophet Yunus (a) left 
his people because the people were really disturbing him.  These people actually saw the punishment coming to them. Just before the punishment striked them, they changed. They realized that ‘Indeed Prophet Yuus (a) was telling us the truth. We must repent.’ As a result of their repentance, they were saved. 
Verse 54: The urge you to hasten the punishment. And hell will 
certainly encompass the disbelievers. The disbelievers are asking for the adhaab because they think it will come from the heavens. But the reality is that they are already in the adhaab. They may not realize it because externally they cannot see what is surrounding them. In reality, they are already in the fire of hell. In the Aakhirat, they will actually be surrounded by the fire.