I have no expericence with chunker or Jottacloud, but --transfers 100 would cause very serious throttling by OneDrive - and thereby effectively reduce the transfer speed.
This thread has some extra info on my experiences with OneDrive.
This help article suggest that Jottacloud could have similar (fair usage) limitations The limitations are most likely per account (like OneDrive) and then it doesn't help to use chunker or increase transfers.
Do you have recent tests/statistics to validate that your change of --transfers improves your transfer speed? (and not the opposite)
What is your upload speed if you do a plain (no extra flags/options):
rclone copy I:\FolderWith10FilesOf1GB jottacloud:FolderWith10FilesOf1GB