We Built a Cache That Made the System Slower

A team added Redis caching to speed up a slow API endpoint, but response times got worse.

Situation

You're a senior engineer at a SaaS company. The dashboard API endpoint takes 4 seconds under load, serving 50,000 transaction rows to compute metrics. Customer complaints are escalating, and your largest enterprise account is threatening to churn.

Stakes

Key API endpoint serving the product's main dashboard was slow under load
Customer complaints escalating — largest enterprise account threatened to churn
Team spent 3 weeks building a caching layer that made the problem worse

Your tech lead proposes adding a Redis caching layer to cache database query results. It'll take about a week to implement. What's your first move?

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