This question is about an actual project that you have done. But the trick to answer is that you are not just telling about the project. You are telling a story. You have to skip the unimportant details of the project.
You have to frame a story out of an actual project that you have worked on. It should start with a problem, challenge and then solution. You have to also mention what exactly you did to solve the problem.
Sample answer is:
In my company, our application for Billing was reaching facing performance problems. There was a high sale season approaching. So we took on a project to resolve performance problems and make the application scalable.
In this project the main challenge was to handle the high load of high sale season. I was responsible for redesigning the application. I used Apache Kafka to create a streaming system. With this requests were coming in asynchronous mode and we were able to handle a large amount of load in short period of time. Once requests were recorded, these were processed by multiple consumer applications.
During the processing logic, we did profiling of code to find the bottlenecks. We used JProfiler to perform profiling. I used caching to resolve the performance improvement.
After the completion of project we were able to handle 1 million requests in less than 1 hr. The application became highly scalable and it is able to handle the load for next 10 years. It was well appreciated by business partners and I received CTO award for doing this project.