Request latency includes queueing, dependencies and local work. Event-loop delay is one signal, not a complete explanation.
Before you start
You should know JavaScript promises, asynchronous errors and the distinction between a process and a request. When following a server example, identify the resource owner and the point where work completes. Try experiments locally with bounded input instead of assuming production traffic behaves like a single request.
The practical goal is to reason through this situation: Correlate p95 latency with event-loop delay, CPU and upstream timing. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Measure distributions
Compare median, p95 and p99 latency rather than relying only on an average.
Step 2: Attribute request time
Collect queue, dependency and local-processing spans alongside CPU and event-loop delay.
Step 3: Repeat representative load
Use realistic concurrency and input sizes, then compare the same conditions after changing one suspected bottleneck.
Worked scenario
Correlate p95 latency with event-loop delay, CPU and upstream timing.
A p95 spike with low event-loop delay and long database spans suggests a different bottleneck from a CPU spike delaying all callbacks. Correlation guides the next experiment but does not prove cause. Include errors and throughput in results so apparently faster responses caused by rejecting work are not mistaken for an improvement.
Common mistake
Average latency can hide a small but serious slow-request population.
Verify the behavior
Compare distributions, throughput and failures over equivalent workloads. Check whether the proposed change improves the actual constrained phase.
Interview exercise
Locate a bottleneck.
Answer and reasoning
Compare distributions and trace spans under representative load before applying optimization or increasing concurrency.
Continue learning
Compare the scenario with the Node.js interview questions and test your understanding with the Node.js MCQs. For terminology and implementation details, consult the reference material.