Network Performance
RTT, bandwidth, loss, retransmission, connection and TLS setup, queueing — and why a request is slow while the server CPU sits idle.
Round-trip time spans five orders of magnitude from loopback to cross-continent, every request costs at least one of them and a cold HTTPS request three or four, so a chatty pattern that is invisible in one data centre is a multi-second disaster across an ocean.
Bandwidth is how many bytes a second the pipe carries; latency is how long the first byte takes; transfer time is latency plus size over bandwidth, and a single TCP connection cannot use a fat, long pipe unless its window covers the bandwidth-delay product.
Throughput is work per unit time in whichever unit you are limited by — requests, packets or bytes — and it is tied to latency by Little’s law and bounded by loss and RTT through TCP’s congestion control, so measuring one without the other tells you very little.
A request’s wall-clock time is a sequence of segments — DNS, TCP, TLS, request, queueing in every buffer along the way, server work, response, retransmissions — and most of them can be large while the server’s CPU is idle; label each one and the slow request explains itself.