Having recently spent several weeks installing gabions as part of our coastal sea defences, I can attest that the time-consuming part was filling the cages in such a way as to minimise settling and slumping, particularly relevant as they are bashed by north-sea waves daily.
Unlike some of our neighbours (which have sagged and bulged) I filled by hand, placing flat-edged stoned to the sides and angled irregular ones to the centre, infilling with smaller stones to fill gaps. I also installed back-to-front bracing/tensioning cables at regular intervals - roughly every 50cm.
After lacing up the cages (they arrived flat-packed), filling them & lacing closed my hands were wrecked!