From testing a couple years ago (things are likely different now), image pull/setup made a pretty noticeable difference. A 1GB container was about 20 seconds slower than a 500MB one--I assume I/O since Fargate instance size didn't make a difference
On the other hand, ECS still seems slow compared to k8s where things are nearly instance unless you're measuring so ECS control plane speed might be part of the issue, too
This is still a thing, Fargate pull times are super slow: https://github.com/aws/containers-roadmap/issues/696. We run all of our workloads on fargate, and it's really annoying when you're trying to iterate on something and you have to sit there waiting on "Provisioning..." for 1-2 minutes every time you launch a task. I don't think the control plane is that slow, as EC2 based ECS launches tasks really fast if the images are already cached on the machine.
People have mentioned image loading but one other shockingly slow thing is allocating ENIs (this also affects Lambda, VPC endpoints, etc.). I've had a few times where I've looked at the logs and it's basically been like 5 minutes to launch something where 4 of those were waiting for the ENI.
