S3 SLOW_DOWN Error and Partitioning Questions

If you're doing multipart uploads, you're probably doing a much larger number of UploadPart operations for each object you upload -- if you weren't, using the multipart mechanism would just slow you down by requiring three API calls instead of just one PutObject. The number of UploadPart calls should be the file size divided by the chunk (part) size, rounded up to the next integer.

You can reduce the number of API calls for multipart uploads by increasing the chunk size. For example, the AWS CLI uses a default chunk size of 8 MiB, although automatically adjusted to a higher value to avoid exceeding the 10,000-part maximum for a multipart upload. For moderately-sized objects, simply increasing the chunk size from 8 MiB to 64 or 128 MiB, for example, would decrease the number of API calls proportionately. Since you're seeing the issue in practice, you might want to give that a go.