Humidity screws with it a lot, when its raining or misty my thick plant fibre slings can become very heavy and stiff. The thinner ones also soak up and become more stiff but the performance is much less affected by it.

also, sisal slings can take a lot of set from for example being in a pocket. To remove this and straighten them before slinging I put my foot in the pouch and give a strong pull on both cords with two hands