The S3 service running on Atmos hardware was written as a compatibility layer on top of the native Atmos API service, so it will be inherently slower.  To what degree largely depends on the workload and usage pattern.  Generally speaking, we recommend that anyone creating new applications choose the S3 API because of its proliferation (it is the most popular object protocol by a large margin).  However, as I mentioned, you may see a slight performance drop on Atmos hardware.  So you have to decide what is most important to you.  If you are planning on migrating to ECS, definitely pick S3.  If you have a large scale application and do not foresee moving to another platform any time soon, pick Atmos REST and use objectspace (Atmos-generated object IDs).  Note that ECS also supports this API, so migrating is still possible.  If you decide to use S3, there are a few things you can check that may improve performance.  First, if you don't need traffic encryption, you might consider using HTTP instead of HTTPS (S3 runs on port 8080 on Atmos).  Next, definitely check that you have HTTP keep-alive enabled on the server and client.  If you're using a load balancer, make sure it is enabled there as well.  This is especially important when using SSL/HTTPS.  If you still have performance issues or further questions, reply here or feel free to email the customer advisory team as well (ecs.customer.advisory.team[NAME].com).
I am evaluating use of S3 vs. REST to interface to ATMOS and see a significant differences. The S3 API uses jets3t and HTTPS. The REST API uses HTTP.  1000 files (all 100 bytes each) archived onto the same EMC ATMOS via: S3:      1061 MS per file REST:  179 MS per file  Is that expected? Has anyone experienced a different performance and/or has suggestions for improvements?
S3 vs. REST performance