I Thought Async Would Make My (Fast) API Faster. I Was Wrong.

When I started learning FastAPI, I kept hearing one thing:

“It’s fast because it supports async.”

So naturally, I assumed:

async = faster API

That assumption was wrong.

What I Initially Believed

I thought that if I changed:

def get_user():

to

async def get_user():

my API would magically handle requests faster.

Spoiler: It doesn’t work like that.

The Real Difference: Concurrency vs Speed

Async does not make your code run faster.

It allows your application to handle multiple tasks efficiently when they are waiting on something.

Think about this:

Your API receives a request and:

• Calls a database

• Calls another service

• Reads a file

• Waits for a network response

While waiting, your server is doing… nothing.

That’s where async helps.

What Async Actually Improves

Async improves performance when:

• You have I/O-bound operations

• Your API waits for database responses

• Your API calls external services

• Your API handles many concurrent users

It does NOT improve performance for:

• Heavy CPU computations

• Data processing

• Large calculations

For CPU-heavy tasks, async gives no benefit.

Why FastAPI Uses Async

FastAPI is built on top of Starlette and uses ASGI instead of traditional WSGI.

ASGI allows handling multiple connections concurrently using an event loop.

So when one request is waiting for a database, another request can be processed.

That’s concurrency.

Not raw speed.

What Finally Clicked for Me

Async is not about making one request faster.

It’s about not wasting time while waiting.

That mental shift changed everything.