Node.js makes single-threaded asynchronous I/O cheap. It also makes a single bad pattern in one corner of the codebase capable of slowing the whole process. This is the production-debugging checklist I'd actually run, in the order I'd run it, with the OpenTelemetry instrumentation that lets you skip the guesswork.
1. The event loop is blocked
The single most common cause of "Node is slow." A CPU-heavy synchronous operation (a regex, a JSON.parse on a 50MB string, a crypto operation) holds the event loop and every other request waits. Symptoms: latency on unrelated endpoints spikes simultaneously, and the spike correlates with one particular endpoint's traffic.
OTel signal: nodejs.eventloop.delay.p95 and nodejs.eventloop.delay.p99via the
@opentelemetry/instrumentation-runtime-node` package. Anything above 20ms is suspect; above
