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Alex Aslam
Alex Aslam

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How We Slashed AWS Lambda Cold Starts by 90% with Node.js

#webdev #programming #javascript #beginners

The Cold Start Nightmare

Our serverless API was fast—when warm. But the moment a new Lambda instance spun up? 1.5-second delays during cold starts. Users noticed. Complaints rolled in.

After weeks of testing, we reduced cold starts from 1500ms to under 150ms—without changing our runtime. Here’s how.

1. The Prime Suspect: Bloated Dependencies

Problem:

Our node_modules was 120MB (!), mostly from:

  • Unused libraries (leftover from old features)
  • Heavy SDKs (AWS SDK v2, full lodash)

Fix:

  • Tree-shaking with esbuild: 
  esbuild src/handler.js --bundle --minify --platform=node --outfile=dist/handler.js
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  • Switched to AWS SDK v3 (modular imports): 
  import { DynamoDBClient } from "@aws-sdk/client-dynamodb"; // 5KB vs 50MB!
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Result: 40% smaller deployments → faster Lambda init.

2. Lazy-Loading for the Win

Problem:

We were initializing everything at startup:

const db = new Database(); // 🐌 Loaded even if unused!
exports.handler = async () => { ... };
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Fix:

Delay non-critical work until first execution:

let db;
const getDB = () => db || (db = new Database()); // Initialize on demand

exports.handler = async () => {
  const database = getDB(); // Only loads when needed
};
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Result: Cold starts 30% faster (less to initialize).

3. Provisioned Concurrency: The Nuclear Option

Problem:

Even after optimizations, sporadic cold starts still happened.

Fix:

Pre-warm Lambdas with provisioned concurrency:

resource "aws_lambda_provisioned_concurrency_config" "api" {
  function_name = aws_lambda_function.api.name
  qualifier     = "prod"
  provisioned_concurrent_executions = 5 # Always keep 5 warm
}
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Cost: ~$15/month (vs. $500+ in lost revenue from slow responses).

Result: Near-zero cold starts for active users.

Key Takeaways

✔ Shrink deployments (esbuild + SDK v3)
✔ Lazy-load everything (faster init)
✔ Pre-warm with provisioned concurrency (if budget allows)

Our API now stays fast—even at 3 AM.

What’s your cold-start war story? Let’s swap fixes!

Further Reading

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