فَأَلْحَقْنَا بِالْهَادِيَاتِ وَدُونَهُ جَوَاحِرُهَا فِي صَرَّةٍ لَمْ تُزَيَّلِ
We caught up with the leading animals of the herd on this horse, while the lagging ones remained behind him, amid a commotion in which the group had not yet broken apart.
Al-hādiyāt are the leaders of the herd, and jawāḥiruhā its stragglers. The speed is conveyed by two relative positions: the leaders are reached and the stragglers are left behind.
Timing is important as well: the horse reaches the herd while the group is still largely together rather than after it has dispersed.