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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