Welcome to your interview.
The question is on your right: Improve System Performance and Reliability. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
You shipped a system that works functionally, but usage has grown and you now need to improve how it performs under load, how reliably it behaves in production, and how smooth the experience feels to users.
How do you optimize for performance, reliability, or user experience in a system you built?