Thanks for the interesting example of the time taken in the preparation of cassava leaves. In the case of cassava leaves, grinding has a purpose, as it removes cyanogens, which are often present in high concentrations.

The grinding thus prevents cyanide poisoning.

A recent report of a simpler preparation method also refers to the literature on nutrient content and preparation of cassava leaves:

Bradbury JH, Denton IC (2011) Mild methods of processing cassava leaves to remove cyanogens and conserve key nutrients. Food Chemistry 127: 1755-9.

Cassava roots may also contain high concentrations of cyanogens. 

For both cassava roots and leaves, removal of cyanogens is an important part of food preparation.