In a service that fans out to many components, rare per-node slowness (GC pauses, contention, a failing disk) is near-certain to hit some node on every request. So tail latency (p99, p99.9) governs the user experience even when medians look healthy.
Then
Set SLOs on p99/p99.9, not the mean, and deploy tail-tolerance — hedged requests, micro-partitioning — instead of trying to make every node uniformly fast.
Unless
Redundancy Has a Price — hedged and tied requests add load; don't chase the tail where the median is what users feel
A leaf that is slow one request in a hundred is invisible on its own. Fan out to 100 of them and the same leaf-level p99 becomes the median user experience: 63% of requests now wait on a straggler. Tail latency is not a per-node property at scale — it is an arithmetic consequence of the fan-out.