فَلَمَّا رَأَوْهُ زُلْفَةً سِييَتْ وُجُوهُ الَّذِينَ كَفَرُوا
...
But when they will see it approaching, the faces of those who disbelieve will be sad and grievous.
means, when the Day of Judgement begins, and the disbelievers witness it, and they see that the matter has drawn close, because everything that is to occur will occur, even if it takes a long time. So, when what they denied occurs, it will grieve them because they will know what is awaiting them there (in Hell) of the worst evil.
This means that it will surround them and Allah will command what they were not able to reckon or fully comprehend to come to them.
وَلَوْ أَنَّ لِلَّذِينَ ظَلَمُواْ مَا فِى الاٌّرْضِ جَمِيعاً وَمِثْلَهُ مَعَهُ لاَفْتَدَوْاْ بِهِ مِن سُوءِ الْعَذَابِ يَوْمَ الْقِيَـمَةِ وَبَدَا لَهُمْ مِّنَ اللَّهِ مَا لَمْ يَكُونُواْ يَحْتَسِبُونَ
وَبَدَا لَهُمْ سَيِّيَاتُ مَا كَـسَبُواْ وَحَاقَ بِهِم مَّا كَانُواْ بِهِ يَسْتَهْزِءُونَ
and there will become apparent to them from Allah what they had not been reckoning. And the evils of that which they earned will become apparent to them, and that which they used to mock at will encircle them. (39:47,48)
This is why it will be said to them in order to scold and reprimand them,
...
وَقِيلَ هَذَا الَّذِي كُنتُم بِهِ تَدَّعُونَ
This is what you were calling for!
meaning, that you were seeking to hasten.