In her short career as one of the world’s leading philanthropists, MacKenzie Scott has made a mark through the enormous scale of her giving and also through its speed, donating nearly $6 billion of her fortune this year alone. Ms. Scott, an author who was once married to Jeff Bezos, the founder and chief executive of Amazon, announced in a Medium post on Tuesday that she had given nearly $4.2 billion to 384 organizations in just the last four months. “Economic losses and health outcomes alike have been worse for women, for people of color, and for people living in poverty. 