Welcome to your interview.
The question is on your right: Background Threading and UI Smoothness. 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.
Explain how background threading works on your preferred platform and how you ensure smooth UI rendering during heavy data processing.