Serverless v2: Are Cold Starts Finally Solved? (2025 Deep Dive)

Serverless v2: Are Cold Starts Finally Solved? (2025 Deep Dive)

"Serverless is great, except for the Cold Starts."

This refrain has haunted the serverless landscape for a decade. The promise of an API that scales to zero and drastically reduces costs is enticing, but the reality of a 3-second delay for the first user often breaks the deal. This latency stems from the infrastructure tax: finding a server, downloading code, booting a MicroVM (Firecracker), starting the runtime, and finally executing the handler.

In 2025, however, the game has fundamentally changed. We have entered the era of Serverless v2.

Powered by innovations like V8 Isolates, Application Snapshots (SnapStart), and specialized JavaScript Runtimes like LLRT, the ecosystem is evolving rapidly. But is the cold start truly dead? We dive into the benchmarks to find out.

Part 1: The Physics of "Booting"

To fix the problem, we must dissect it. A Cold Start consists of:

1. Infrastructure Init: Booting the VM. (AWS responsibility).
2. Runtime Init: Starting Node/Java. (Language responsibility).
3. Function Init: Importing libraries, connecting to DB. (Developer responsibility).

Infrastructure Init is now incredibly fast (<100ms) thanks to Firecracker. The bottleneck today is Runtime Init. Initializing a heavy framework like Spring Boot or Express takes time.

Part 2: The Solutions (Hardware & Software)

Solution 1: Memory Snapshots (AWS SnapStart)

Used primarily for Java. Instead of booting the JVM every time:

1. AWS boots your function once during deployment.
2. It runs the initialization code.
3. It takes a Snapshot of the memory (RAM).
4. It saves this to disk.

When a user request comes in:

1. AWS restores the RAM from disk.
2. Execution resumes instantly.

Result: Cold starts drop from 3000ms to 200ms.

Solution 2: V8 Isolates (Cloudflare Workers / Deno Deploy)

This is a radical architecture shift. Instead of starting a Linux Container (MicroVM) for every customer, they run One Huge Process (V8 Chrome Engine). Each customer gets an Isolate (a lightweight context).

• Overhead: <5ms.
• Cold Start: Effectively 0ms.

You are not renting a computer. You are renting a function call inside a shared computer. Constraint: It is not Node.js. You can't use C++ bindings. You have strict CPU limits.

Solution 3: Specialized Runtimes (LLRT)

AWS introduced LLRT (Low Latency Runtime). It is a JavaScript runtime built in Rust specifically for Lambda. It strips out the JIT compiler (Just-In-Time).

• Node.js optimizes for long-running code (JIT makes it fast after warmup).
• Serverless code runs for 50ms. JIT is useless overhead. LLRT starts 10x faster than Node.js.

Part 3: Runtime Benchmarks (2025)

Scenario: Hello World API Gateway Trigger.

Winner: Cloudflare Workers for Edge. Rust/LLRT for AWS Lambda.

Part 4: Architecture Patterns (Developer Duty)

Even with SnapStart, you can ruin performance with bad code.

1. Lazy Loading

Don't write:

const aws = require('aws-sdk'); // Loads huge library globally
exports.handler = ...

Write:

exports.handler = async () => {
   const aws = require('aws-sdk'); // Loads only when needed
   ...
}

Actually, in 2025 the V3 SDK is modular, so this matters less, but the principle stands.

2. Database Proxies & Pooling

Connecting to Postgres takes 200ms (SSL Handshake). If you do this inside the handler, every cold start pays this tax. Solution:

• Define the client outside the handler (Global Scope). The container might be reused (Warm Start).
• Use RDS Proxy or Prisma Data Proxy. They maintain a pool of warm connections so your Lambda connects instantly over HTTP/TCP.

3. Provisioned Concurrency

The "Money Solve". You pay AWS to keep 10 instances "Warm" 24/7. This defeats the cost benefits of Serverless (Scale to Zero), but guarantees performance. Use this for the "Checkout" flow, but not for the "About Us" page.

Conclusion: The Problem is Solved (If you choose correctly)

If you write Rust or stick to Cloudflare Workers, cold starts are effectively gone. If you stick to Node/Python, they are manageable (<500ms). If you use Java without SnapStart, you are hurting your users.

Serverless is no longer just for cron jobs. In 2025, it is the default backend for latency-sensitive APIs, provided you pick the modern runtime stack.

Recommendation

• API: Cloudflare Workers (Hono framework).
• Backend Logic: AWS Lambda (Node.js with LLRT or Rust).
• Heavy Compute: Standard Containers (Fargate).