Response model exclude and include in FastAPI - Performance & Optimization

Performance: Response model exclude and include

MEDIUM IMPACT

This affects the size of the JSON response sent to the client, impacting network load and client rendering speed.

Sending API responses with only necessary fields

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    id: int
    username: str
    email: str
    password: str

@app.get('/user', response_model=User, response_model_exclude={'password'})
async def get_user():
    user = User(id=1, username='alice', email='[e-mail]', password='secret')
    return user

Excludes password field from response, reducing payload size and improving security.

📈 Performance GainSmaller JSON response reduces network transfer and speeds up LCP.

Sending API responses with only necessary fields

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    id: int
    username: str
    email: str
    password: str

@app.get('/user', response_model=User)
async def get_user():
    user = User(id=1, username='alice', email='[e-mail]', password='secret')
    return user

Returns all fields including sensitive data like password, increasing response size and security risk.

📉 Performance CostAdds unnecessary data to response, increasing payload size and slowing LCP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full response with all fields	N/A	N/A	Higher due to larger JSON parsing	[X] Bad
Response with exclude/include to limit fields	N/A	N/A	Lower due to smaller JSON parsing	[OK] Good

Rendering Pipeline

The response model exclude/include affects the JSON serialization stage before sending data to the client. Smaller JSON means less data to parse and render on the client side.

→JSON Serialization

→Network Transfer

→Client Rendering

⚠️ BottleneckNetwork Transfer due to payload size

Core Web Vital Affected

LCP

This affects the size of the JSON response sent to the client, impacting network load and client rendering speed.

Optimization Tips

1Always exclude sensitive or unnecessary fields to reduce response size.

2Use include to send only the fields the client needs.

3Smaller JSON responses improve network transfer and page load speed.

Performance Quiz - 3 Questions

What is the main performance benefit of using response_model_exclude in FastAPI?

A Adds more fields to the response

B Reduces response JSON size, improving load speed

C Increases server CPU usage

D Improves database query speed

DevTools: Network

How to check: Open DevTools, go to Network tab, make the API request, and inspect the response payload size and content.

What to look for: Look for smaller response size and absence of unnecessary fields in JSON to confirm good performance.