Every system has a bottleneck. Find it before your interviewer does:
CPU-bound: Processing takes too long. Solution: optimize code, add more compute, parallelize.
Memory-bound: Not enough RAM for working set. Solution: add memory, reduce data size, use disk.
I/O-bound: Waiting for disk or network. Solution: caching, SSDs, reduce round trips.
Network-bound: Bandwidth or latency limits throughput. Solution: compression, CDN, batch requests.
In interviews, after drawing your design, ask: "Where's the bottleneck?" Then proactively address it. This shows you think about real-world constraints.