async

How FastAPI works under the hood: Uvicorn, then Starlette, then Pydantic, then your function on the event loop or the threadpool

How FastAPI Works Under the Hood (I Tested Every Claim)

by Dimuthu Harshana
September 23, 2026September 23, 2026

Same code: 10 seconds with async def, 1 second with def. How FastAPI works under the hood, with Uvicorn, Starlette and Pydantic measured on FastAPI 0.141.