gsutil cp hangs on many small files when running in parallel

I have a GCS bucket with millions of small files in different folders. When I run:

```bash
$ gsutil -m cp -r gs://my-bucket .
```
The process will eventually hang before completion, sometimes after 5 minutes and sometimes after several hours. This seems to be is 100% reproducible. I'm using version 4.27 but this has happened in older versions as well. As a workaround have I to use:

```bash
$ gsutil cp -r gs://my-bucket .
```

which works but it takes several days to download everything so it's not optimal.