Performance New Added 28 August 2026

Node.js: high latency from synchronous crypto or JSON on the event loop

Node runs JavaScript on a single thread. A synchronous bcrypt hash, a large JSON.parse, or a synchronous file read blocks every other request for its full duration, which shows up as latency on unrelated endpoints.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Measure first
const { monitorEventLoopDelay } = require('node:perf_hooks');
const h = monitorEventLoopDelay({ resolution: 10 }); h.enable();
setInterval(() => console.log('p99 lag ms', h.percentile(99) / 1e6), 5000);

// Move CPU work off the loop
const { Worker } = require('node:worker_threads');
// or use the async APIs: bcrypt.hash (callback/promise), not hashSync
// and fs.promises.readFile, not fs.readFileSync

// For very large JSON, stream it rather than parsing in one call.

How to diagnose Performance errors

Performance failures rarely produce an error message. They produce timeouts elsewhere. The three classic causes are stop-the-world garbage collection, thread pool starvation (all workers blocked on I/O so new requests queue), and event loop blocking in single-threaded runtimes. All three look identical from outside: latency climbs, then upstream timeouts fire. Distinguishing them requires looking inside the process.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Measure the p99, not the mean. Averages hide exactly the pauses that cause timeouts.
For the JVM, enable GC logging (-Xlog:gc*) and correlate pause durations with latency spikes before tuning anything.
For thread pools, log active versus queued task counts. A queue that grows while CPU is idle is starvation, and the fix is asynchronous I/O, not more threads.
In Node.js, measure event loop lag directly (perf_hooks.monitorEventLoopDelay). Any synchronous work over a few milliseconds per request will show up here.
Profile before optimising. pprof, async-profiler, py-spy and Chrome's profiler all point at the real hot path, which is rarely where intuition suggests.

Tools worth reaching for

async-profiler
go tool pprof
py-spy
perf_hooks.monitorEventLoopDelay
-Xlog:gc*

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

nodejs.org

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