The answer
A measure of how far behind the event loop is. If it is consistently behind, something is blocking it with synchronous code. Event loop lag is a key production metric for Node.js performance, since it directly reflects responsiveness.
How to Debug V8 and Node.js Performance Issues
What is event loop lag in Node.js?
More FAQs in How to Debug V8 and Node.js Performance Issues
Use the CPU profiler to find the bottleneck, --trace-deopt to check for deoptimization, heap snapshots for memory issues, event loop monitoring for blocking code, clinic.js for blocking detection, and --trace-warnings for performance warnings.
Run with the --trace-deopt flag. This shows deoptimization events, telling you if your code is hitting the optimized path or falling back to interpreted bytecode. It reveals why a function is slower than expected.
Take heap snapshots with the Node.js inspector. Compare snapshots over time to find growing objects, which point to memory leaks or excessive allocation. This is the standard way to debug memory issues in production.
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