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Lambda cold starts increase to 8–12 seconds after migrating to Node.js 20

Fresh6 months ago

Mar 14, 20260 views

Confidence Score65%

65%

Problem

After migrating AWS Lambda functions from Node.js 18 to Node.js 20, INIT_DURATION in CloudWatch logs jumps from ~400ms to 8–12 seconds. Warm invocations are normal speed. The regression is caused by Node.js 20 changes to module loading combined with webpack-bundled code that initializes all modules eagerly, including heavy AWS SDK v2 imports that are not tree-shakeable.

Error Output

INIT_DURATION: 11423.45 ms
DURATION: 234.12 ms
MAX_MEMORY_USED: 312 MB

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1 Fix

Canonical Fix

Moderate Confidence Fix

64% confidence50% success rate3 verificationsLast verified Mar 14, 2026

Switch to esbuild bundling and tree-shake AWS SDK v3

Medium Risk

Webpack bundles include all imported modules eagerly. AWS SDK v2 is not tree-shakeable. Moving to esbuild with AWS SDK v3 per-client imports and --external:@aws-sdk/* (excluding the runtime-available SDK) reduces bundle size and init time dramatically.

Trust Score

3 verifications

50% success

Switch from AWS SDK v2 to v3 per-client imports

Replace monolithic SDK import with specific clients:

typescript

// ❌ SDK v2 — not tree-shakeable, large bundle
import AWS from 'aws-sdk'
const s3 = new AWS.S3()

// ✅ SDK v3 — tree-shakeable, import only what you need
import { S3Client, GetObjectCommand } from '@aws-sdk/client-s3'
const s3 = new S3Client({ region: 'us-east-1' })

Bundle with esbuild and exclude AWS SDK

AWS SDK v3 is available in the Lambda runtime — exclude from bundle:

bash

npx esbuild src/handler.ts \
  --bundle --platform=node --target=node20 \
  --external:@aws-sdk/* \
  --minify --outfile=dist/handler.js

Move SDK client initialization outside handler

Module-level code runs once per container, not per invocation:

typescript

// Module level — runs once (warm invocations reuse this)
const s3 = new S3Client({ region: 'us-east-1' })

export const handler = async (event) => {
  // s3 already initialized
}

Validation

Check INIT_DURATION in CloudWatch Logs Insights. Should drop from 8–12s to under 500ms.

Verification Summary

Worked: 3

Failed: 3

Last verified Mar 14, 2026

Environment

Product: AWS Lambda
Version: Node.js 20
Environment: production

Submitted by

Alex Chen

2450 rep