Sending and receiving messages in FastAPI - Performance & Optimization

Performance: Sending and receiving messages

MEDIUM IMPACT

This affects how quickly messages are sent and received over the network, impacting server response time and user interaction speed.

Handling message sending and receiving in a FastAPI app

FastAPI

from fastapi import FastAPI
import asyncio
app = FastAPI()

@app.post('/send')
async def send_message(message: str):
    # Simulate async processing
    await asyncio.sleep(0.1)
    return {'status': 'sent'}

Using async def and non-blocking await allows server to handle other requests concurrently, improving responsiveness.

📈 Performance GainNon-blocking, reduces INP, improves server throughput

Handling message sending and receiving in a FastAPI app

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.post('/send')
def send_message(message: str):
    # Simulate heavy processing
    import time
    time.sleep(5)
    return {'status': 'sent'}

Blocking the request with time.sleep delays response, causing slow user experience and server thread blocking.

📉 Performance CostBlocks server worker for 5 seconds, increasing INP and reducing throughput

Performance Comparison

Pattern	Server Blocking	Concurrency	Response Time	Verdict
Synchronous blocking with time.sleep	High (blocks worker)	Low (serial requests)	Slow (delays response)	[X] Bad
Asynchronous with await asyncio.sleep	Low (non-blocking)	High (handles many requests)	Fast (quick response)	[OK] Good

Rendering Pipeline

Message sending and receiving in FastAPI affects the server's ability to quickly process requests and send responses, which impacts the time until the browser can update UI after user interaction.

→Server Processing

→Network Transfer

→Browser Rendering

⚠️ BottleneckServer Processing when blocking synchronous code is used

Core Web Vital Affected

INP

This affects how quickly messages are sent and received over the network, impacting server response time and user interaction speed.

Optimization Tips

1Avoid blocking calls like time.sleep in FastAPI endpoints.

2Use async def and await for non-blocking message processing.

3Monitor request durations in DevTools Network tab to catch slow responses.

Performance Quiz - 3 Questions

What is the main performance problem with using time.sleep in a FastAPI message handler?

A It causes layout shifts in the browser

B It blocks the server worker, delaying other requests

C It increases network latency

D It reduces CSS selector efficiency

DevTools: Network

How to check: Open DevTools, go to Network tab, send a message request, and observe the time taken for the request to complete.

What to look for: Look for long request durations indicating blocking; shorter times indicate better async handling.