This thread was about moving data ("File Move speed is slow"), which would be an instant rename operation if the source and destination were the same device, so I did not even consider copying from and to the same NAS.
Explorer (via the Shell file-copy API) may be telling the NAS to do the copy itself, without any of the data going over the network at all. What does Task Manager's network bandwidth graph show while the copy is happening in Explorer? I believe there is a very low-level API for doing that with newer versions of SMB (or iSCSI apparently*) if the server supports it.
(*Edit: From doing some research, SMB and iSCSI both have different but similar features which were recently added to do this, circa WIndows 8.)
That is not something Opus or many other programs support, but you can call the shell file copy APIs from Opus in cases where you want do that kind of operation, which would give you the same result as in Explorer or other programs that use those APIs. You'd lose extra functionality when doing that, but most of the extra functionality would not be compatible with a file copy which the NAS was basically performing by itself anyway;