Refactor & Optimize MERN Application

I have a live MERN stack product that is starting to feel sluggish and dated. I want to run a focused optimisation and refactor sprint that touches two areas only: the React-based frontend and the MongoDB layer... (Budget: $25 - $50 USD, Jobs: AngularJS, Backend Development, Frontend Development, JavaScript, MERN, MongoDB, Node.js, NoSQL Couch & Mongo)