System Latency Budget Calculator

A latency budget is the maximum acceptable response time for a system operation. It's important because it ensures consistent user experience, sets performance goals for development teams, helps identify bottlenecks before they impact users, and enables data-driven performance decisions. Without latency budgets, systems often degrade slowly until users notice and leave.

P95 (95th percentile) means 95% of requests complete within that time. We use P95 instead of average because it captures the long tail of slow requests that users actually experience. An average of 200ms might sound good, but if 5% of users wait 2 seconds, those users will have a poor experience. P95 ensures we optimize for the majority while still paying attention to edge cases.

Start with user expectations: 100ms for instant feel, 100-300ms for fast, 300-500ms for acceptable. Then subtract: Frontend (200ms), Backend (200ms), Network (200ms), Database (100ms) = 700ms total. Monitor actual performance and adjust based on real data. Consider your specific use case - checkout pages might need stricter budgets than search results.

Track all components that contribute to the user's perceived response time: DNS lookup, TCP connection, TLS handshake, server processing, database queries, API response, content download, JS parse/execute, render/paint. These are the building blocks that make up your P95 target - if any component is over budget, it contributes to the overall latency.

The traffic impact figures are illustrative, not measured. The calculator scales an estimated 'slow users' percentage and bounce rate by how far your total current latency overshoots the P95 target. They are meant to communicate relative severity, not predict exact business outcomes. Use real RUM (Real User Monitoring) data for production decisions.

Action items highlight components that exceed their budget. Each item shows what's over budget, by how much (percentage and absolute), and which layer needs optimization. For example: 'Content Download is 50% over budget (+100ms)'. These are your optimization priorities - focus on the largest overshoots for maximum impact.