If it affects both Opus and TeraCopy, and if the problem can be solved by changing something on the NAS side (creating an iSCSI target), then I'd say the problem is in the NAS.
Any program copying files will simply loop reading and writing data as fast as the disk/network will allow. It sounds like the NAS is not accepting data as quickly as it could under certain circumstances. Without the source-code to Explorer (or the NAS) it's hard to guess what the difference between Opus/TeraCopy and Explorer is that is triggering the speed difference.
Yes, you can do that. I've just posted the info here for easy reference: Copy files via the shell (Windows Explorer).