In ceph storage's s3 interface, there is a flag named "allow-unordered" that allows rgw (the gateway) to don't force the cluster to sort all of objects in case of list-objects-v2 requests.
There is also a command option in radosgw-admin (gateway commandline utility) named "--allow-unordered" which provides unordered list to.
It's not officially supported by AWS, but included in S3 services which are based on Ceph storage (for example, DigitalOcean Spaces, Redhat Ceph Storage, Alibaba Cloud OSS, Sotoon S3, ...)

Is there any plan to support unordered list in s3cmd?

Update: I found a way to extend aws-cli's with add this file to ~/.aws/models/s3/2006-03-01/. but not available in s3cmd.