I had a similar task once, tried it with awscli and s3cmd first, then took about 5 minutes to google for a multithreaded alternative and found s4cmd[1], problem solved. (tmux to keep the session running)

Makes me kinda doubt at least parts of this story - not one of these 7 engineers thought about googling?

Edit to add: Agree with the basic premise though - s3 can be real slow when when you actually need to work with your data.

[1] https://github.com/bloomreach/s4cmd