gsutil cp hangs for files over 1MB when transfering to s3 bucket

running the following on a google compute VM just hangs for files over 1MB. No issues with small files (30kb).
```
gsutil cp gs://bucket/subfolder/file s3://bucket/
```
Gsutil version 4.59

The same issue is seen on my local machine running version 4.53

Start by reproducing the `gsutil cp gs://bucket/subfolder/file s3://bucket/` command with a file larger than 1MB, using the reported gsutil versions and both the Google Compute VM and local environment where possible. Trace the `gsutil cp` entry point and compare it with the successful 30KB transfer. Done means cross-cloud copies over 1MB complete instead of hanging, with regression coverage for the reported case.

Written by the indexing model from the issue text.