Fix FastAPI Response Slow

What's Actually Happening

FastAPI endpoints respond slowly, taking seconds to return responses.

The Error You'll See

```bash $ curl -w "@curl-format.txt" http://localhost:8000/api/slow

time_total: 5.234s # Too slow! ```

Why This Happens

1. 1.Blocking I/O - Synchronous operations in async code
2. 2.Database queries slow - N+1 queries, missing indexes
3. 3.External API calls - Slow third-party services
4. 4.No caching - Recomputing every request
5. 5.Heavy serialization - Large response payloads

Step 1: Profile Performance

```python import time from fastapi import FastAPI

app = FastAPI()

@app.middleware("http") async def add_process_time_header(request, call_next): start_time = time.time() response = await call_next(request) process_time = time.time() - start_time print(f"Request took {process_time:.3f}s") return response ```

Step 2: Fix Blocking Operations

```python # BAD: Blocking in async @app.get("/api/slow") async def slow_endpoint(): time.sleep(5) # Blocks event loop! return {"status": "ok"}

# GOOD: Use async operations import asyncio

@app.get("/api/fast") async def fast_endpoint(): await asyncio.sleep(5) # Non-blocking return {"status": "ok"} ```

Step 3: Optimize Database Queries

```python # BAD: N+1 queries @app.get("/users") async def get_users(): users = await db.users.all() for user in users: user.posts = await db.posts.filter(user_id=user.id).all() return users

# GOOD: Eager loading @app.get("/users") async def get_users(): users = await db.users.all().prefetch("posts") return users ```

Step 4: Add Caching

```python from fastapi_cache import FastAPICache from fastapi_cache.decorator import cache

@app.get("/api/cached") @cache(expire=60) async def cached_endpoint(): return {"data": "expensive_computation()"} ```

FastAPI Performance Checklist

Verify the Fix

curl -w "Time: %{time_total}s\n" http://localhost:8000/api/
# Output: Time: 0.150s

Related Issues

• [Fix FastAPI Dependency Injection Failed](/articles/fix-fastapi-dependency-injection-failed)
• [Fix FastAPI Background Task Not Running](/articles/fix-fastapi-background-task-not-running)