The Horde section has far more idle than it seems, especially once you've progressed further. Micro-managing your ability uses can really only get you through an extra few zones, with the impact and frequency of this dropping as the game progresses. Prestige time penalty, corrupted flesh, and monster parts all mean that relying on active play only serves to get you to a time wall sooner. You could instead rely on prestige bonuses for such progress.