There are many different super quick builds for farming Greater Rift Keystones in Torment Nephalem Rifts. The Cain's Destiny set change was a great addition to increase key efficiency. All the top build usually play this set or just switch to it when Killing the Rift Guardian. Below you can find an overview with the current top builds for each class:
Overview
S Tier
101+ KEYS/H
A Tier
UP TO
100 KEYS/H