For the API gateway design, I started with the main functionality: routing requests, user authentication, and basic request logging. I mentioned using tools like Kong or AWS API Gateway and tried to explain how the gateway would handle rate limiting and caching for improved performance.

The interviewer seemed engaged but nudged me to consider how to scale the gateway under heavy load. I stumbled a bit on discussing load balancing and didn't provide concrete examples of what has worked or failed in my past experiences, which I think left some gaps in my explanation.

Overall, I feel I communicated the core idea, but the lack of depth on scaling aspects hurt me.