It is Sunnah for the imam to shorten congregational prayers as there may be weak people in the congregation

The Messenger of Allah ﷺ said: “When one of you leads the people in prayer, let him make it brief, for among them are the weak, the sick, and the elderly. But when one of you prays by himself, let him lengthen it as much as he wishes.”

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «إِذَا صَلَّى أَحَدُكُمْ لِلنَّاسِ فَلْيُخَفِّفْ, فَإِنَّ مِنْهُمُ الضَّعِيفَ وَالسَّقِيمَ وَالْكَبِيرَ, وَإِذَا صَلَّى أَحَدُكُمْ لِنَفْسِهِ فَلْيُطَوِّلْ مَا شَاءَ».