The answer
Slow file operations, crypto, or DNS lookups that improve when you increase UV_THREADPOOL_SIZE. The event loop is not blocked (other requests work), so it is a thread pool bottleneck, not event loop blocking.
How to Debug Thread Pool Issues in Node.js
How do I know if I have a thread pool issue in Node.js?
More FAQs in How to Debug Thread Pool Issues in Node.js
Recognize symptoms (slow fs/crypto, event loop free), check and try increasing UV_THREADPOOL_SIZE, distinguish from event loop blocking (which stops all requests, not just fs/crypto), and consider worker threads for CPU-heavy alternatives.
Thread pool issues do not block the event loop; the main thread is free, but fs/crypto operations are slow because they queue for 4 threads. Event loop blocking stops all requests entirely. If only fs/crypto is slow but other requests work, it is the thread pool.
For CPU-heavy crypto or data processing, consider worker_threads for explicit control. The thread pool is managed internally by libuv with 4 default threads. Worker threads let you scale computation beyond the pool's limits.
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