blocking vs async using guzzle

https://www.youtube.com/watch?v=OEMuHy5Srk8

Here's video of Rasmus explaining how multi-threading works on PHP.

For what I understand from your description, sync or async wouldn't make much difference, because your PHP process on server is running throughout all the subcalls to AWS API.

If you are worried that this once back-end call will take too much resource away from your normal visitors, you should move it to second server instance that does the heavy duty tasks and keep the first server to only handle front end web requests.