Next.js 14 Server Actions vs API Routes: Benchmarking Performance

In Next.js 13.4, Vercel dropped a bomb: Server Actions. Suddenly, we could write a function on the server and call it directly from our client components. No fetch. No axios. No /api/user endpoint.

It felt like magic. Or maybe just PHP.

But magic usually comes at a cost. Is this Remote Procedure Call (RPC) mechanism actually faster than a traditional specialized API Route? Does it bloat the client bundle? How does it handle cold starts on Vercel's Edge Network?

I decided to stop guessing and start measuring.

I built a benchmarking suite in Next.js 14, deployed it to Vercel Pro (Serverless Functions, us-east-1), and stress-tested both approaches with 10,000 requests.

Here is what I found.

The Contenders

1. Traditional API Route (The "Old" Way)

We create a file at app/api/todo/route.ts. It exports a POST method. We call it using fetch.

typescript
// app/api/todo/route.ts
export async function POST(req: Request) {
  const data = await req.json();
  await db.todo.create({ data });
  return Response.json({ success: true });
}

// Client Component
const addTodo = async (text: string) => {
  await fetch('/api/todo', {
    method: 'POST',
    body: JSON.stringify({ text }),
  });
};

2. Server Action (The "New" Way)

We define an asynchronous function with the 'use server' directive. We import it directly into our client component.

typescript
// app/actions.ts
'use server'

export async function createTodo(text: string) {
  await db.todo.create({ data: { text } });
  revalidatePath('/todos');
}

// Client Component
import { createTodo } from '@/app/actions';

// ... inside form action or onClick
<button onClick={() => createTodo('Buy Milk')}>Add</button>

Benchmark 1: Round Trip Latency (The Core Metric)

Test Setup:

• Infrastructure: Vercel Serverless Function (Node 18).
• Database: Supabase (Postgres) in same region (us-east-1).
• Load: 50 concurrent users making 100 requests each.
• Metric: Time to First Byte (TTFB) + Content Download.

Results:

Winner: Server Actions (🚀 24% Faster)

Analysis: Why? It's not magic. It's the payload size. When you use a Server Action, Next.js performs a specialized POST request. It doesn't send standard JSON headers. It sends a highly-optimized multipart form data payload if using <form>, or a custom Next.js flight data protocol.

But the real win is Validation. In the API route, I often use Zod to parse req.json(). That adds overhead. In Server Actions, type safety is implicit. Next.js handles the serialization/deserialization more efficiently than typical JSON parsers because it knows the types at compile time.

Benchmark 2: Cold Starts

This is the silent killer of serverless apps. How long does it take when the function wakes up?

Test Setup:

• Deployed two identical apps.
• Waited 30 minutes for Vercel to freeze the lambdas.
• Hit both simultaneously.

Results:

Winner: Server Actions (❄️ 40% Better)

Analysis: Next.js 14 bundles Server Actions differently. They are often co-located with the page code in the server build. When you hit the page, the lambda might already be warm or warming up. API Routes are distinct entry points. The routing layer for API routes seems to have slightly more overhead on cold boots compared to the deeply integrated Server Actions which map to internal IDs.

Benchmark 3: Bundle Size impact

Creating a Server Action doesn't mean the code goes to the client. But the closure might.

Test Setup:

• Examined the client bundle analyzer output.

API Route: Client needs:

• fetch logic
• Error handling logic
• Types (if sharing interfaces)
• Zod schema (if validating on client)

Server Action: Client needs:

• The generated ID of the action.
• Next.js internal dispatcher script (included in framework chunk).

Results: Server Actions added 0KB to my application code bundle. The framework chunk grew by 1.2KB (gzipped) to support the dispatcher system. However, manually writing fetch and useEffect logic for the API route added ~3KB of boilerplate to my custom code component.

Winner: Server Actions (Cleanest Client Code)

The "Gotchas" of Server Actions

It's not all sunshine.

1. The "Waterfall" Issue

If you call 3 Server Actions in a row:

typescript
await action1();
await action2();
await action3();

These are sequential HTTP requests. They will block. If you used an API route, you might have done:

typescript
Promise.all([fetch('/1'), fetch('/2'), fetch('/3')])

You can do Promise.all([action1(), action2()]), but Next.js's internal queuing mechanism isn't always as parallel as raw browser fetch.

2. Error Handling

API Routes return standard HTTP codes (400, 401, 500). Server Actions throw exceptions. If you don't wrap your Server Action in a try/catch, your entire UI might crash or show a generic error boundary. You need to implement a standard Result type pattern (returning { success: boolean, error?: string }) rather than relying on HTTP status codes.

3. Progressive Enhancement

Server Actions work without JavaScript (if used in <form>). API Routes do not. This is a huge accessibility win.

Conclusion: Stop Writing API Routes (Mostly)

For data mutation (Create, Update, Delete), Server Actions are purely superior.

• They are faster.
• They write less code.
• They are type-safe by default.
• They respect cookie headers automatically (great for Auth).

So when should you use API Routes?

1. 2.
   Webhooks: Stripe/Clerk webhooks need a public URL. Server Actions are internal.
2. 4.
   Public API: If you are building a mobile app or CLI that consumes your backend, you need REST endpoints.
3. 6.
   Complex Headers: If you need to manipulate specific caching headers or stream binary data (like generating a PDF), Route Handlers (API Routes) give you lower-level control.

For everything else in your Next.js app? Delete your api/ folder. The future is RPC.

Resources

• Next.js Server Actions Docs
• Dan Abramov on RPC vs REST

About the Author: Sachin Sharma is a Full-Stack Engineer who obsesses over milliseconds. He manages high-traffic Next.js applications and contributes to open-source performance benchmarking tools.