From Q&A on migrating a 5-year-old Rails+React app. First step is measuring client-side performance — backend-focused tools like Datadog and New Relic aren't enough. Use a Core Web Vitals RUM product (Ryan suggests Cloudflare Web Analytics). Measurement gives ammo to drive change, correlates slowness to churn/conversion/bounce, and lets you iteratively rip out the worst offenders first — starting with wins that are easy or medium-effort, building momentum for harder refactors later.