It is mostly on the destination you can tweak things
--s3-chunk-size SizeSuffix Chunk size to use for uploading. (default 5M)
Increasing that will help - note that the chunks are stored in memory. Rclone will use a chunk size of 60MB to get the number of parts below 10,000 anyway.
--s3-upload-concurrency int Concurrency for multipart uploads. (default 4)
You might want to up this - note that there are this many chunks stored in memory
I recommend the latest beta also - there was a bug in s3 memory usage with big files which is now fixed.