Keep in mind that Duplicati is doing a bunch of file processing & sqlite commands and potentially chopping, hashing, compression, & temp file creation before any actual file transfer takes place.
To compare Duplicati’s speed to a normal file copy you’d have to dig through the logs so you could subtract any time spend doing stuff that’s NOT the actual file copy.
You could get closer to what Duplicati does by writing batch file that compresses what you want copied into a temp file, copies that temp file to the destination, then deletes the temp file - though it still wouldn’t simulate time spent on SQL lookups or hashing…