“Our Standard storage class provides end-to-end p99 latencies of under 500 milliseconds.”
S2 sells streams as a service on top of S3 Standard and a quorum of S3 Express One Zone buckets. Express promises under 50 milliseconds with no disks of its own. It is free during the preview, and the pricing is a promise to come in “meaningfully cheaper.” Your durability is Amazon’s durability with a startup’s API on top.