Aws Lambda Cold Start: Find Exactly What Is Slow With LLRT and Init Tracing

# LLRT (Low Latency Runtime) — released 2024
# Built on QuickJS instead of V8 — no JIT compilation = faster cold start

# Benchmarks vs Node.js 20 (same function, 512MB, us-east-1):
# Hello world:          Node.js 180ms | LLRT 42ms  (77% faster)
# AWS SDK v3 + DynamoDB: Node.js 480ms | LLRT 95ms  (80% faster)
# Express.js API:        Node.js 620ms | LLRT 210ms (66% faster)

# Trade-offs:
# + Cold starts 70-80% faster
# - Warm invocations 20-40% slower (no JIT warm-up)
# - Not all Node.js APIs supported (partial crypto, worker_threads)
# - Still experimental (Q1 2026)

# Deploy with LLRT:
Resources:
  MyFn:
    Type: AWS::Serverless::Function
    Properties:
      Runtime: provided.al2023
      Layers:
        - !Sub arn:aws:lambda:us-east-1:094274105915:layer:LLRTNodejs20:10
      Handler: index.handler

// Wrap each init operation in an X-Ray subsegment
const AWSXRay = require("aws-xray-sdk-core");

const seg = AWSXRay.getSegment().addNewSubsegment("init");

const dbSeg = seg.addNewSubsegment("db-pool");
const pool = await createConnectionPool({ max: 10 });
dbSeg.close();

const schemaSeg = seg.addNewSubsegment("schema-load");
const schema = await loadSchema("/var/task/schema.json");
schemaSeg.close();

seg.close();

// X-Ray now shows:
// init (total: 1,240ms)
//   db-pool: 820ms   <- bottleneck
//   schema-load: 180ms
// Now you know exactly what to optimize

// BEFORE: everything loads during cold start
const sharp = require("sharp");       // 400ms — even for non-image requests
const Jimp = require("jimp");         // 200ms

exports.handler = async (event) => {
  if (event.action === "resize") return resizeImage(event, sharp);
  return processData(event); // Still paid 600ms for unused imports
};

// AFTER: lazy load
let _sharp;
const getSharp = () => _sharp || (_sharp = require("sharp"));

exports.handler = async (event) => {
  if (event.action === "resize") return resizeImage(event, getSharp());
  return processData(event); // Cold start now 400ms faster
};

// AWS SDK v3 — import only what you need (tree-shakeable)
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { GetItemCommand } from "@aws-sdk/client-dynamodb";
// NOT: import * as AWS from "aws-sdk"; // v2 costs 300-500ms

# Cold start distribution:
filter @type = "REPORT" and ispresent(@initDuration)
| stats percentile(@initDuration, 50) as p50,
        percentile(@initDuration, 90) as p90,
        percentile(@initDuration, 99) as p99,
        count() as coldStarts
        by bin(1h)

# Correlation: memory vs cold start duration:
filter @type = "REPORT" and ispresent(@initDuration)
| parse @message "Memory Size: * MB" as memMB
| stats avg(@initDuration) as avgInit by memMB
| sort memMB asc