What missing from description is network setup.
Is it ec2 classic, VPC? Is ec2 getting to s3 through IG? Hopefully not through NAT. There is also VPC endpoint to s3. Which all may have different performance profiles especially with multiple instances.
Network was VPC. The EC2 instance had an IG attached, yes, but I'm not sure if you're asking if an internal vs. external URL for S3 was used? Are you saying there's a better endpoint than s3-<region>.amazonaws.com for S3 requests from EC2?
It's a private connection to AWS services including S3.
You'd use the same URL as it's a routing basically.
No idea if VPC endpoints would be better than IG though.
P.S. Just tested and I get about half of the latency on VPC endpoint.
