MERN App Bug Fixing

My production MERN application needs a thorough round of bug-squashing. I am seeing performance slow-downs on key screens, functional errors that break expected user flows, and a few security weak points that were flagged in a recent audit... (Budget: ₹1500 - ₹12500 INR, Jobs: AngularJS, Full Stack Development, JavaScript, MERN, MongoDB, Node.js, NoSQL Couch & Mongo, React.js)