HTTP/3 promises better performance, but deploying it at scale on Android reveals hidden trade-offs. At DeliveryHero, we rolled out HTTP/3 (QUIC) across Android apps, serving millions of users in 18 European and APAC markets. Using a randomized controlled experiment, we measured a 51% reduction in connection handshake latency and up to 17% faster p90 home screen response times, alongside unexpected regressions on some secondary screens. This talk shares the complete journey: from identifying last-mile mobile networking as our dominant performance bottleneck, through deploying HTTP/3, to diagnosing why certain optimizations didn't materialize. You'll learn how we navigated real-world challenges, including Cronet's initialization penalty, Cloudflare's congestion-control choice that degraded throughput under packet loss, JNI myths, and more. Attendees will leave with a clear, practical framework for determining when HTTP/3 is appropriate and how to roll it out safely on Android today.