TL;DR Event loop must never block.  Sync file reads, heavy CPU work, and long loops freeze all requests. Use async APIs ( fs.promises ), offload CPU to worker threads, and chunk large arrays with  setImmediate . Clustering utilizes all CPU cores.  Node.js single-thread leaves cores idle. Use  cluster  module or PM2 ( pm2 start app.js -i max ). Requires stateless design – move sessions and caches to Redis. Redis caching > in-memory. node-cache  works per worker but not shared.  ioredis  provides shared cache across processes and servers, plus persistence and pub/sub. Monitor event loop lag, heap usage, and active handles.  Use Clinic.js for profiling ( clinic doctor -- node app.js ). Set  NODE_ENV=production  for framework optimizations. Common fixes: Promise.all()  for parallel I/O, stream large files (avoid  fs.readFileSync ), set heap limits ( --max-old-space-size=4096 ), and implement graceful shutdown for zero-downtime deploys.…