MobX is a reactive state management library that is presented as an alternative to Redux. It aims to have less boilerplate code than Redux and be simpler to use. MobX uses observable objects and computed values, along with the @observer decorator for React components. According to a performance test, MobX had faster performance than Redux for common operations like adding, removing, and updating items in a Todo list app with 10,000 items. MobX is praised for its flexibility and less code compared to Redux, but it also lacks strict best practices and conventions that Redux provides.