Why middleware processes requests globally in FastAPI - Performance Evidence

Performance: Why middleware processes requests globally

MEDIUM IMPACT

Middleware affects the time it takes for every request to be processed before reaching route handlers, impacting overall request latency.

Adding logging or authentication checks to requests

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def fast_middleware(request, call_next):
    # Non-blocking async operation example
    response = await call_next(request)
    return response

Avoids blocking calls and runs lightweight code, minimizing delay on all requests.

📈 Performance GainReduces request delay to near zero, improving INP and user experience.

Adding logging or authentication checks to requests

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def slow_middleware(request, call_next):
    import time
    time.sleep(0.1)  # blocking delay
    response = await call_next(request)
    return response

Blocking operations in middleware delay every request, causing slow response times.

📉 Performance CostBlocks rendering for 100ms on every request, increasing INP significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking middleware with sleep	N/A	N/A	Delays response arrival	[X] Bad
Non-blocking async middleware	N/A	N/A	Minimal delay on response	[OK] Good

Rendering Pipeline

Middleware runs before route handlers, so it affects the server's response time which impacts how quickly the browser receives data to render.

→Request Processing

→Response Generation

⚠️ BottleneckMiddleware blocking or slow operations delay the entire request lifecycle.

Core Web Vital Affected

INP

Middleware affects the time it takes for every request to be processed before reaching route handlers, impacting overall request latency.

Optimization Tips

1Middleware runs on every request, so keep it fast and non-blocking.

2Avoid heavy or blocking operations inside middleware to prevent slowing all requests.

3Use async code in middleware to improve server responsiveness and user experience.

Performance Quiz - 3 Questions

Why does middleware affect all requests in FastAPI?

A Because middleware only runs on error responses

B Because middleware runs globally on every request before route handlers

C Because middleware runs after the response is sent

D Because middleware only runs on static files

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and check the Time column for request delays.

What to look for: Look for high waiting (TTFB) times indicating slow server response caused by middleware.