Users say the orders page is slow. How do you find and fix the problem across frontend, API and database?

What the interviewer wants to hear

• Measure first: browser DevTools, Core Web Vitals, network waterfall
• Check API timing with logs, tracing or APM
• Inspect slow queries with EXPLAIN ANALYZE and add indexes
• Look for N+1 queries, large payloads and missing pagination
• Fix, then measure again and add monitoring

Model answer

I start by measuring, not guessing. In Chrome DevTools I look at the network waterfall and performance panel: is the time spent waiting for the API, downloading a large JavaScript bundle, or rendering a huge list? If the API is slow, I check server logs or tracing with something like OpenTelemetry to see which part of the request takes time. Often it is the database, so I run EXPLAIN ANALYZE on the slow query, look for sequential scans and add a suitable index, or fix an N+1 pattern by using a join. I also check if the API returns too much data, and add pagination or select only needed columns. On the frontend I might virtualize a long list, split a heavy bundle or cache data with a library like TanStack Query. After each fix I measure again and add an alert so we notice if it gets slow later.

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