I've not done any measurements in terms of latency but from my experience an agent on the host will ask the docker daemon to start a new container in the order of seconds after making an API request to ECS that calls for an additional container to be provisioned.
I'd recommend setting up a private registry within EC2 though, otherwise you'll have a fairly significant delay while it pulls the image from wherever and incur bandwidth charges for doing so. You'll still have some delay while it pulls the image from your registry running on EC2 but not as significant if it were externally hosted.
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I've not done any measurements in terms of latency but from my experience an agent on the host will ask the docker daemon to start a new container in the order of seconds after making an API request to ECS that calls for an additional container to be provisioned.
I'd recommend setting up a private registry within EC2 though, otherwise you'll have a fairly significant delay while it pulls the image from wherever and incur bandwidth charges for doing so. You'll still have some delay while it pulls the image from your registry running on EC2 but not as significant if it were externally hosted.