You may get better results by experimenting with the Copy Buffer size in Opus. Settings -> Preferences / File Operations / Copying Files, at the bottom. I would try much bigger values first but it could be smaller ones that make it word. It's difficult to predict.
If that doesn't help, check the version of Samba on the server if you haven't already. Sometimes you end up with a very old one which can cause problems.
Failing everything else, you could create buttons (or drag & drop context menus or whatever you want) in Opus which copy the files using the DOS copy command, or a tool like TeraCopy or RoboCopy.