Diagnose why a scaled system became slow

You are on-call for a production service that recently scaled up (more instances, more users/traffic). After the scale-up, users report the system is “much slower” (higher latency, timeouts), even though the service is still functional. Design a practical, step-by-step troubleshooting approach to identify the bottleneck(s) and stabilize the system. Cover at least: - What metrics and dashboards you would check first (client, load balancer, service, downstream dependencies). - How you would isolate whether the issue is CPU, memory/GC, disk I/O, network, database, cache, or a specific dependency. - How you would use logs, tracing, and profiling to narrow it down. - Immediate mitigations vs. longer-term fixes. - Common “scaled-up system got slower” root causes (e.g., thundering herd, connection pool saturation, cache miss storms, lock contention, hot partitions).