If one request makes 100 calls and each is slow 1% of the time, 1 - 0.99^100 ≈ 63% of requests see at least one slow call.
Tail latency
The slowest few percent of requests (p99, p999), which dominate what users feel in fan-out systems.
If one request makes 100 calls and each is slow 1% of the time, 1 - 0.99^100 ≈ 63% of requests see at least one slow call.