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Node.js Performance

Event Loop Lag

Detect main-thread blocking, the most common Node latency killer.

lag

Detailed Description

Node.js runs JavaScript on a single main event loop. If synchronous work blocks that loop, every request, timer, callback, and promise continuation waits behind it.

Event-loop lag is one of the most important Node-specific signals. High lag often points to CPU-heavy loops, synchronous crypto, huge JSON parsing, large array transforms, or garbage collection pressure.

Mental Model

set timer
-> expected time
-> actual time
-> lag = drift

When To Use

  • ->Detect CPU-bound synchronous work
  • ->Catch large JSON.parse or crypto blocking
  • ->Alert before users see timeouts
Tools: perf_hooks monitorEventLoopDelay, timer drift detector
event-loop-lag.js

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Output

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