`gcloud storage rsync` is slower than `gsutil rsync` when

Intuitively, these steps are functionally equivalent.
In our experiments, the new method is ~12x slower. Is there a configuration problem?

deprecated builder:

```
- name: 'gcr.io/cloud-builders/gsutil'
args:
- '-m'
- 'rsync'
- '-d'
- '-r'
- '-c'
- 'dags/'
- '${_COMPOSER_BUCKET}/dags'
```

new method:

```
- name: 'gcr.io/google.com/cloudsdktool/cloud-sdk:slim'
args:
- 'storage'
- 'rsync'
- '--checksums-only'
- '--delete-unmatched-destination-objects'
- '--recursive'
- 'dags/'
- '${_COMPOSER_BUCKET}/dags'
```

Reproduce the two Cloud Build configurations using the provided gsutil rsync and gcloud storage rsync commands, then compare their execution behavior and timing. Determine whether the differing flags or configuration explain the reported slowdown; done means documenting the cause or confirming the equivalent configuration needed for comparable performance.

Written by the indexing model from the issue text.