You're taking about Opus doing something special to run slower with this device at the other end, but we're probably really talking about everything except the Windows shell file copy API.
Opus doesn't do anything that special, at least that the device at the other end of the network should care about. It reads from the source file into a buffer and writes that buffer to the destination file. If non-buffered IO is enabled then separate threads are used for the read and write ends (and if it isn't, then the OS itself should do something similar to cache ahead of the read position).
Some devices are only tuned to the exact nuances of how the shell file copy API works (e.g. particular buffer sizes, maybe, although I can only guess) and run slowly when everything else sends data to them.
Such devices will be slow with everything else. That includes e.g. saving files to the network share using software, like saving an image in Photoshop or a video in Premier.
At least based on the info we know so far, the question is why is the server slow when anything but Explorer (or other things using the shell filecopy API, which is very limited but also can be faster with some devices that aren't tested properly with anything else). From what you've said, Opus isn't a required factor in seeing slow transfers to the device, since you saw them with another program as well. I'm betting you'll see them with most programs.
(It's also worth noting the usual things about measuring file copy performance: Buffering can impact things greatly if doing the same copy more than once. Antivirus can slow down one program or method of copying while ignoring another. Explorer's progress dialog says it is complete abd closes a long time before it really is complete, while data is still in write buffers. Different programs calculate speed differently. Buffer sizes can also affect some network (and USB, as an aside) devices very differently. Those things may not explain the whole difference between Explorer and the other two programs you've tested, but may account for some of it. Opus being configured in a way that causes it to read data from the share or access it in other ways at the same time it is being witten is abother potential factor; Process Montior could reveal that kind of thing.)