In the world of Python web development, two frameworks have become go-to tools for building APIs — Flask and FastAPI. Flask has long been favored for its simplicity and flexibility, making it ideal for beginners and quick prototypes. On the other hand, FastAPI has rapidly gained popularity for its blazing speed, async support, and automatic API documentation. But when it comes to performance, scalability, and modern development standards, how do they really compare?

In this blog, will break down the core differences between Flask and FastAPI — from architecture to execution — and back it up with a live benchmark test using real code. Whether you’re building a small application or planning a large-scale API system, this comparison will help you choose the right tool for the job.

What Are Flask and FastAPI?

Flask

Flask is a micro web framework written in Python. It’s minimalistic and doesn’t enforce project structure or dependencies. It has been around since 2010 and is trusted by large companies for its simplicity and flexibility.

Key Features:

• Simple and lightweight
• Synchronous (blocking) by default
• Large plugin ecosystem
• Extensive documentation

FastAPI

FastAPI is a modern, fast (high-performance) web framework for building APIs with Python 3.7+ using asyncio and type hints. It was designed to support asynchronous programming out of the box, making it ideal for high-performance APIs.

Key Features:

• Asynchronous and non-blocking
• Built-in support for type checking and data validation
• Automatically generates OpenAPI docs
• Fast execution and better concurrency

Flask vs FastAPI: Key Differences

1. Speed: Flask is synchronous and blocking, which makes it slower for handling multiple requests. FastAPI is built with async support, making it significantly faster and more efficient under load.
2. Async Support: Flask has limited support for asynchronous code. FastAPI, on the other hand, is designed with async/await in mind, making it a better choice for high-performance, concurrent applications.
3. API Documentation: In Flask, documentation must be manually created using tools like Swagger. FastAPI automatically generates interactive API docs (Swagger UI and ReDoc) from your code and type annotations.
4. Ease of Use: Flask is extremely beginner-friendly and ideal for quick prototypes or simple apps. FastAPI is also developer-friendly but has a slightly steeper learning curve due to its use of async and modern Python features.
5. Performance: Flask handles requests one at a time (unless explicitly made multi-threaded), which limits concurrency. FastAPI supports handling many requests concurrently, resulting in better throughput and responsiveness.

Real-World Comparison: FastAPI Beats Flask in Performance

To compare both frameworks fairly, have created two minimal applications exposing a /ping endpoint.

Setup:

• 1-second sleep to simulate a delay
• Both endpoints respond with JSON
• Benchmarked using httpx with 100 concurrent requests

FastAPI Code Snippet:

Flask Code Snippet:

Benchmark Script:

A simple Python script was used to send 100 asynchronous GET requests and measure total execution time.

Results

When running the benchmark against both servers:

fastapi:

flask:

FastAPI handled 100 requests in just 1.26 seconds, while Flask took a much longer 100.8 seconds for the same load.

Why this matters:

• FastAPI used asyncio.sleep(), allowing other requests to be processed while one sleeps.
• Flask blocked every request due to time.sleep(), processing them sequentially.
• This real-world test highlights how FastAPI handles concurrency far more efficiently, making it ideal for high-load applications.

Conclusion:

Both Flask and FastAPI are powerful frameworks, but they serve different needs. Flask is great for simplicity and quick development, while FastAPI stands out with its speed, async support, and modern features. If performance and scalability are your priorities, FastAPI is the clear winner.

Sanjay N