Single file upload is not using entire upload speed

welcome to the forum,

as a windows user, i never use rclone to access smb.
i use windows os native tools, might get better performance and bonus, checksum verification of file transfers.

check out my howto guide https://forum.rclone.org/t/how-to-access-smb-samba-with-rclone/42754

imho, accessing smb direct over the internet is not safe.
smb was not designed to be used over slow, high latency networks such as the internet.
so i would test webdav or maybe sftp