Try tweaking these. Larger chunks will be faster but use more memory. Disabling the checsum will stop a pause at the start and upload concurrency may help.
--s3-chunk-size int Chunk size to use for uploading (default 5M)
--s3-disable-checksum Don't store MD5 checksum with object metadata
--s3-upload-concurrency int Concurrency for multipart uploads (default 2)