Great question — there are a few common bottlenecks between “worker submitted an ECS task” and “container is actually running.” Here are the best levers to reduce that latency for Prefect 3.x ECS work pools (Fargate-focused): Image and code delivery (usually the biggest win) - Use a small, prebuilt image: Multi-stage builds, slim base (e.g., python:3.x-slim), no compilers, strip caches. Keep the final image as small as possible. - Bake your flow code and dependencies into the image: Avoid

pip install

or

git clone

at container start. In Prefect 3.x, deploy with an image that already contains your code and point your deployment’s

entrypoint

at it so there’s no source retrieval at runtime. - Host the image in ECR in the same region as your ECS cluster. - Pin images by digest to avoid metadata round-trips on “latest” and ensure consistent layer resolution. ECS networking and placement - Subnets/IP capacity: Ensure your task can get an ENI quickly. - Provide multiple subnets with plenty of free IPs in your ECS job config. - If you’re in private subnets, IP scarcity or NAT congestion can add seconds. - Assign public IP (when acceptable): If your task must reach the internet (e.g., to pull from ECR without VPC endpoints),

assignPublicIp: ENABLED

in public subnets can avoid NAT bottlenecks. - VPC endpoints: If running in private subnets, add interface endpoints for ECR (api + dkr) and CloudWatch Logs to bypass NAT and speed pulls and log stream creation. - https://docs.aws.amazon.com/AmazonECR/latest/userguide/vpc-endpoints.html - Security groups: Keep them simple and avoid anything that blocks ECR/CloudWatch connectivity. Task definition and runtime settings - Fargate platform version: Use the latest (1.4.x) — it improves image pull and networking behavior. - https://docs.aws.amazon.com/AmazonECS/latest/developerguide/platform_versions.html - CPU/memory sizing: Bigger CPU can speed your app’s startup after the image pulls; it won’t fix placement, but it helps time-to-ready once the container is scheduled. - Log group pre-creation: Pre-create the CloudWatch log group and ensure permissions are correct. Letting ECS auto-create can add seconds on first runs. - Capacity providers: If you’re using FARGATE_SPOT, occasional placement delays can happen. For fastest, most consistent starts, stick to FARGATE (on-demand). - Task definition reuse: If you’re dynamically registering TDs on each run, consider reusing a fixed Task Definition ARN to skip registration overhead. Prefect-specific considerations - ECS work pool job variables: Set image, CPU, memory, subnets, security groups, assignPublicIp, platformVersion, task/execution roles, and logs config in the base job template so every run is ready-to-go with no dynamic setup. - Source retrieval: Prefer deployments that don’t fetch code at runtime (i.e., code is in the image and

entrypoint

points to your flow). This removes git download time and any dependency install. - Worker location: Run the worker in the same region as your ECS cluster to minimize API latency. Ensure the worker has adequate IAM to submit tasks without retries. When you need even faster starts - Consider ECS on EC2 with pre-warmed instances that already have your image pulled. Fargate does not cache images between tasks; EC2 can, which dramatically reduces startup time if you can manage that infra. (This is more ops-heavy, but it’s the biggest lever if you need sub-10s cold starts.) Useful references - Prefect 3.x ECS guide: https://docs-3.prefect.io/integrations/prefect-aws/ecs_guide - AWS ECS task lifecycle: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_life_cycle.html - AWS blog – reducing ECS/Fargate startup time: https://aws.amazon.com/blogs/containers/reducing-startup-time-on-amazon-ecs-and-aws-fargate/ - ECR VPC endpoints: https://docs.aws.amazon.com/AmazonECR/latest/userguide/vpc-endpoints.html If you share a bit more detail, I can give targeted steps:

- Are you using Fargate or EC2 launch type? - Rough image size and where hosted? - Is your flow code baked into the image or pulled at runtime (from Git or storage)? - In ECS events, do you see long time in PENDING/PROVISIONING (placement/networking) or RUNNING but app slow to start (image/app init)?