Why not just rely on server-side metrics (CPU, GC, qps)?

Because users don’t feel CPU. They feel LCP, INP, and checkout latency. Server metrics are necessary but insufficient. We’ve seen servers look healthy while RUM showed LCP p75 blowing past 3s on mid Android due to a font swap and 3rd-party tags.

Synthetic or RUM—do I need both?

Yes. Synthetic (Lighthouse, k6) is fast and deterministic for CI gating. RUM proves impact on real devices, networks, and geos, and powers SLO-based canaries and rollbacks.

Won’t performance tests slow down CI?

Run quick smoke checks (30–90 seconds) on PRs and deeper tests nightly. Gate only critical routes/APIs per PR; everything else can be async. The cost of a perf regression in prod is higher than a 2–3 minute CI job.

How do I pick budgets?

Start with Google’s Web Vitals guidance (LCP <2.5s, INP <200ms) and your current p75/p95. Set budgets 10–20% tighter than current, then ratchet down after each win. Make them per route, per device class, and per region.

What about microservices backends?

Budget at the edge of the user journey. Then add contracts for hot APIs (p95/p99 by region). Use k6 with tags per endpoint, and validate the journey in canary with RUM. Don’t drown in 200 service-level charts—users don’t care which microservice was slow.

Performance-optimization · Nov 6, 2025 · 10 minute read

The Optimization Isn’t Real Until CI Says So: Automating Performance Proof with User-Centric Metrics

Q: What about microservices backends?

Budget at the edge of the user journey. Then add contracts for hot APIs (p95/p99 by region). Use k6 with tags per endpoint, and validate the journey in canary with RUM. Don’t drown in 200 service-level charts—users don’t care which microservice was slow.

If it doesn’t move LCP, INP, and checkout latency for real users, it’s not an optimization. Wire performance tests into CI/CD and gate merges with budgets tied to revenue.

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The Optimization Isn’t Real Until CI Says So: Automating Performance Proof with User-Centric Metrics

Key takeaways

Implementation checklist