Boost FastAPI Response Speed 5× with a One‑Line ORJSON Switch

FastAPI is known for speed and ease of use, but you can make its API responses even faster with just one line of code by optimizing JSON serialization.

This article shows how swapping FastAPI’s default JSON serializer for a faster, more efficient one can significantly improve response time and reduce server load without rewriting existing endpoints.

JSON Serialization Bottleneck

When FastAPI generates responses, it uses Python’s built‑in json module to serialize objects. While functional, this module isn’t optimized for high throughput or large data structures, leading to unnecessary latency and higher CPU usage for complex or large JSON payloads.

Simple Change to Boost Speed

Replace the default serializer with a faster one that offers:

Extremely fast serialization performance

Native support for dataclasses and NumPy types

Automatic handling of dates and times

Compact, smaller JSON output

Strict Unicode and JSON compliance

You can increase API response speed up to five times.

Below is a comparison chart of json vs ORJSON:

How to Apply the Change in FastAPI

Integration is straightforward. After installing the faster JSON library, set FastAPI’s default response class:

from fastapi import FastAPI
from fastapi.responses import ORJSONResponse

app = FastAPI(default_response_class=ORJSONResponse)

@app.get("/data/")
async def get_data():
    return [{"id": i, "value": f"Item {i}"} for i in range(1000)]

That’s it! Your API now uses the ultra‑fast serializer under the hood.

Why It Matters

Faster serialization means:

Lower response latency, improving user experience.

Reduced server CPU usage, allowing more requests per resource.

Smaller load, saving bandwidth and speeding up transfers, especially on slower networks.

Better handling of complex data types without manual workarounds.

Optimizing FastAPI responses is one of the simplest yet most effective improvements you can make, unlocking significant performance gains without added complexity.

If performance and scalability are important for your API, try this change—you’ll be surprised how much impact a single line of code can have.

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