Why should a composed endpoint call multiple downstream services concurrently instead of one after another?

Calling services sequentially means the total response time is the sum of every individual call's time — if four services each take 100 milliseconds, a sequential composer takes close to 400 milliseconds. Calling them concurrently, for example using Mono.zip with Spring's reactive WebClient, fires all the requests at essentially the same instant and waits only as long as the slowest single call, bringing that same scenario down to roughly 100 milliseconds. For a screen where a real user is actively waiting for a response, that difference is often the gap between an interface that feels instant and one that feels noticeably slow, even though both approaches fetch exactly the same data from exactly the same services.