Bun vs. Node.js in 2025: Is the "Cold Start" Speed Worth the Stability Risk?

 

The Hook: The Latency vs. Reliability Paradox

In 2025, the "Serverless Cold Start" is still the primary villain of backend performance. You are likely facing a familiar dilemma: your Node.js Lambda functions or containerized microservices take 600ms–1.5s to boot, resulting in unacceptable P99 latency spikes.

Bun enters the room claiming sub-50ms startup times, powered by JavaScriptCore and Zig. The benchmarks look incredible. Your CTO wants to switch immediately.

However, you are hesitant. Node.js has 15 years of edge-case handling in V8 and libuv. Bun, while maturing, still exhibits unpredictable behavior with specific native bindings, subtle discrepancies in node:stream implementation, and occasional segmentation faults in long-running processes.

You do not need to rewrite your infrastructure in Bun to solve cold starts. The solution isn't swapping runtimes; it's changing how you compile Node.js.

The Why: Anatomy of a Cold Start

To solve this, we must understand where the time goes during a Node.js cold start. It isn't the execution of your logic; it is the Bootstrap Phase.

When a standard Node.js instance boots:

1. V8 Initialization: The engine spins up, allocates the heap, and initializes the garbage collector.
2. Module Resolution: Node traverses node_modules, reading package.json files to resolve dependency trees.
3. Parsing & Compilation: V8 parses JavaScript text into an Abstract Syntax Tree (AST), generates bytecode, and eventually optimizes hot paths (JIT).
4. Execution: Finally, your code runs.

Bun is fast because it uses a binary format for modules and JavaScriptCore, which is optimized for faster startup at the cost of peak sustained throughput.

In 2025, the correct engineering approach for stability-critical applications is to eliminate steps 2 and 3 in Node.js using V8 User Startup Snapshots.

The Fix: Implementing V8 Startup Snapshots

Instead of migrating to Bun, we will implement V8 User Startup Snapshots. This feature allows us to initialize our application, load all dependencies into memory, and then take a binary snapshot of the heap. When the production process starts, it loads this binary blob, instantly restoring the application state as if it had already booted.

We will use Node.js 22+ (LTS).

1. The Application Structure

Imagine a standard API service (app.ts) that imports heavy libraries (like Zod or AWS SDK).

// app.ts
import { createServer, IncomingMessage, ServerResponse } from 'node:http';
import { z } from 'zod'; // Heavy validation library
import { S3Client } from '@aws-sdk/client-s3'; // Heavy SDK

// Simulate heavy initialization logic that usually kills cold start
const heavySchema = z.object({
  id: z.string().uuid(),
  payload: z.string().min(100),
});

// Pre-allocate expensive clients OUTSIDE the request handler
const s3 = new S3Client({ region: 'us-east-1' });

let server: any;

// This function handles the actual request
const requestHandler = (req: IncomingMessage, res: ServerResponse) => {
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({ status: 'warm', time: Date.now() }));
};

// We wrap the listen command so it's not triggered during snapshot generation
export const start = () => {
  server = createServer(requestHandler);
  server.listen(3000, () => {
    console.log('Server listening on port 3000');
  });
};

2. The Snapshot Entry Point

We need a dedicated entry point that determines if we are building the snapshot or running from it.

// entry.ts
import { start } from './app.js';
// specific Node.js API for interacting with the snapshot feature
import { isBuildingSnapshot } from 'node:v8'; 

if (isBuildingSnapshot()) {
  // PHASE 1: BUILD TIME
  // During snapshot generation, the 'import' statements in app.ts 
  // have already executed. The S3Client is allocated. The Zod schemas are parsed.
  // The Heap is populated.
  
  // We attach a lifecycle hook to run when the snapshot is deserialized
  // @ts-ignore - v8.startupSnapshot is available in recent Node versions
  v8.startupSnapshot.setDeserializeMainFunction(() => {
    // This runs instantly at runtime
    console.log('Heap deserialized. Starting server...');
    start();
  });

} else {
  // PHASE 2: FALLBACK / DEV MODE
  // If not using snapshots (e.g., local dev), just start normally.
  start();
}

3. The Build Configuration

We need to compile the TypeScript and then generate the snapshot blob.

// package.json (Partial)
{
  "type": "module",
  "scripts": {
    "build": "tsc",
    "snapshot": "node --snapshot-blob snapshot.blob --build-snapshot dist/entry.js",
    "start:fast": "node --snapshot-blob snapshot.blob dist/entry.js"
  }
}

4. Running the Solution

Execute the following in your CI/CD pipeline or Dockerfile:

# 1. Transpile TS to JS
npm run build

# 2. Generate the heap snapshot (This happens at build time, not runtime)
# This executes the global scope of your modules, serializes the memory,
# and saves it to 'snapshot.blob'.
npm run snapshot

# 3. Start the server (Production Runtime)
# This skips parsing and module resolution entirely.
npm run start:fast

The Explanation

Why does this bridge the gap between Bun and Node?

1. Serialized Heap vs. Parsing: When you run start:fast, Node.js does not parse your dist/entry.js or node_modules. It simply mmaps the snapshot.blob file directly into memory. The V8 Isolate is "hydrated" with your S3Client and Zod schemas already instantiated.
2. JIT Preservation: The snapshot can contain code usage data. While it's not fully JIT-compiled code, the shapes of objects (Hidden Classes) are already established, preventing deoptimization penalties that usually happen in the first few seconds of execution.
3. Security & Stability: You are still running on the V8 engine. You retain access to widely tested debugging tools, APMs (Datadog/NewRelic), and native bindings that might segfault in Bun.

The Trade-off

There is one catch: Environment Variables. Because the heap is initialized at build time, process.env captured during the build is baked into the blob.

To fix this, you must access configuration dynamically inside the setDeserializeMainFunction or your request handler, ensuring you read the runtime environment variables, not the build-time ones.

// Correct pattern for Env Vars with Snapshots
v8.startupSnapshot.setDeserializeMainFunction(() => {
    // Read ENV vars here, at runtime boot
    const dbUrl = process.env.DATABASE_URL; 
    connectToDb(dbUrl); 
    start();
});

Conclusion

The debate between Bun and Node.js often ignores the capabilities of modern Node. Bun is an excellent tool for local development, scripting, and edge routers where logic is minimal. However, for complex backend architectures involving heavy SDKs, ORMs, and critical business logic, the stability risk of a newer runtime is high.

By utilizing V8 Startup Snapshots, you achieve startup performance comparable to Bun (often reducing cold starts from 1s to <100ms) without sacrificing the maturity and stability of the Node.js ecosystem.

Stick with Node for the runtime, but stop booting it like it's 2015.