Request timing middleware in FastAPI - Performance & Optimization

Performance: Request timing middleware

MEDIUM IMPACT

This affects the server response time and perceived page load speed by measuring how long each request takes to process.

Measuring request processing time without blocking response

FastAPI

from fastapi import FastAPI, Request
import time
import asyncio

app = FastAPI()

@app.middleware("http")
async def timing_middleware(request: Request, call_next):
    start = time.time()
    response = await call_next(request)
    end = time.time()
    duration = end - start
    # Schedule logging asynchronously without blocking
    asyncio.create_task(async_log(duration))
    return response

async def async_log(duration: float):
    print(f"Request took {duration:.4f} seconds")

Logging is done asynchronously after response is sent, so it does not delay the client.

📈 Performance GainNon-blocking logging saves ~1-5ms per request, improving server response time

Measuring request processing time without blocking response

FastAPI

from fastapi import FastAPI, Request
import time

app = FastAPI()

@app.middleware("http")
async def timing_middleware(request: Request, call_next):
    start = time.time()
    response = await call_next(request)
    end = time.time()
    duration = end - start
    # Blocking logging inside middleware
    print(f"Request took {duration:.4f} seconds")
    return response

Logging inside middleware blocks the response until logging completes, adding latency to every request.

📉 Performance CostBlocks response for logging duration, increasing server response time by ~1-5ms per request

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking synchronous logging in middleware	0	0	0	[X] Bad
Asynchronous logging after response	0	0	0	[OK] Good

Rendering Pipeline

Request timing middleware runs during the server's request handling phase before sending the response. It measures processing time but does not affect browser rendering directly.

→Server Processing

→Response Delivery

⚠️ BottleneckBlocking synchronous operations inside middleware delay response delivery.

Core Web Vital Affected

LCP

This affects the server response time and perceived page load speed by measuring how long each request takes to process.

Optimization Tips

1Avoid blocking operations inside request timing middleware to keep server response fast.

2Use asynchronous tasks for logging or heavy work after sending the response.

3Monitor server response times in DevTools Network tab to detect middleware delays.

Performance Quiz - 3 Questions

What is the main performance risk of synchronous logging inside request timing middleware?

A It blocks the response, increasing server response time.

B It causes layout shifts in the browser.

C It increases the bundle size significantly.

D It reduces the number of DOM nodes.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and check the Time column for server response duration.

What to look for: Look for lower server response times indicating faster middleware processing.