FastAPI uvicorn app start-up is very slow, causing timeouts

[tibbe]

We have a FastAPI app running under uvicorn that's very slow to start-up, causing the Lambda to even hit its 29s timeout. Is it possible from the logs below to tell which part is slow?

timestamp,message
1762529077933,"INFO lambda_web_adapter: app is not ready after 2000ms url=http://127.0.0.1:8080/health"
1762529079767,"INFO lambda_web_adapter: app is not ready after 4000ms url=http://127.0.0.1:8080/health"
1762529081770,"INFO lambda_web_adapter: app is not ready after 6000ms url=http://127.0.0.1:8080/health"
1762529083768,"INFO lambda_web_adapter: app is not ready after 8000ms url=http://127.0.0.1:8080/health"
1762529085657,"EXTENSION	Name: lambda-adapter	State: Ready	Events: []"
1762529085657,"INIT_REPORT Init Duration: 9930.03 ms	Phase: init	Status: timeout"
1762529087721,"INFO lambda_web_adapter: app is not ready after 2000ms url=http://127.0.0.1:8080/health"
1762529089722,"INFO lambda_web_adapter: app is not ready after 4000ms url=http://127.0.0.1:8080/health"
1762529091721,"INFO lambda_web_adapter: app is not ready after 6000ms url=http://127.0.0.1:8080/health"
1762529093745,"INFO lambda_web_adapter: app is not ready after 8000ms url=http://127.0.0.1:8080/health"
1762529095721,"INFO lambda_web_adapter: app is not ready after 10000ms url=http://127.0.0.1:8080/health"
1762529097716,"INFO lambda_web_adapter: app is not ready after 12000ms url=http://127.0.0.1:8080/health"
1762529099712,"INFO lambda_web_adapter: app is not ready after 14000ms url=http://127.0.0.1:8080/health"
1762529101714,"INFO lambda_web_adapter: app is not ready after 16000ms url=http://127.0.0.1:8080/health"
1762529103721,"INFO lambda_web_adapter: app is not ready after 18000ms url=http://127.0.0.1:8080/health"
1762529104252,"{""level"":""INFO"",""location"":""_serve:83"",""message"":""Started server process [13]"",""timestamp"":""2025-11-07 15:25:04,252+0000"",""service"":""service_undefined"",""taskName"":""Task-1"",""color_message"":""Started server process [\u001b[36m%d\u001b[0m]""}"
1762529104281,"{""level"":""INFO"",""location"":""startup:48"",""message"":""Waiting for application startup."",""timestamp"":""2025-11-07 15:25:04,261+0000"",""service"":""service_undefined"",""taskName"":""Task-1""}"
1762529104281,"{""level"":""INFO"",""location"":""startup:62"",""message"":""Application startup complete."",""timestamp"":""2025-11-07 15:25:04,262+0000"",""service"":""service_undefined"",""taskName"":""Task-1""}"
1762529104322,"{""level"":""INFO"",""location"":""_log_started_message:215"",""message"":""Uvicorn running on http://127.0.0.1:8080 (Press CTRL+C to quit)"",""timestamp"":""2025-11-07 15:25:04,322+0000"",""service"":""service_undefined"",""taskName"":""Task-1"",""color_message"":""Uvicorn running on \u001b[1m%s://%s:%d\u001b[0m (Press CTRL+C to quit)""}"
1762529104348,"{""level"":""INFO"",""location"":""route_handler:31"",""message"":""Received request"",""timestamp"":""2025-11-07 15:25:04,348+0000"",""service"":""service_undefined"",""taskName"":""starlette.middleware.base.BaseHTTPMiddleware.__call__.<locals>.call_next.<locals>.coro""}"
1762529104361,"INFO:     127.0.0.1:60672 - ""GET /health HTTP/1.1"" 204 No Content"

[bnusunny]

Your FastAPI app took more than 10 seconds to start. Lambda restarted the runtime at 9930 ms and re-initialize the runtime.

1762529085657,"INIT_REPORT Init Duration: 9930.03 ms	Phase: init	Status: timeout"

The app took more than 18 seconds to answer http requests.

1762529103721,"INFO lambda_web_adapter: app is not ready after 18000ms url=http://127.0.0.1:8080/health"

You can reduce the dependencies imported during the startup, or use Lambda SnapStart. Lambda SnapStart might be easier option to reduce the cold start time.

Checkout the Lambda Developer Guide and the launch blog post.