Suggested approach: This is a tricky question to answer because “performance” has many forms, so be prepared to adjust your answer as you talk based on interviewer feedback.
If they mean graphical performance then you should probably talk about using the Core Animation instrument to identify slow drawing, but you should also consider venturing into the cost of Auto Layout in things like table view cells where lots of work happens quickly.
If they mean code performance then perhaps talk about stack traces, retain cycles, unnecessary caches, and similar – again make sure and bring Instruments in.
And then there's network performance, where you might talk about things such as batching requests to reduce battery wastage or using compression to save bandwidth.
Estimated difficulty: Advanced
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