What kind of NAS is it?
Have you checked for firmware updates or reports of similar problems (slow speed when using things other than Explorer)?
Have you checked in case your antivirus or firewall is treating Opus differently?
Which buffer settings did you try changing, and what to?
Have you tried any other programs to see what their speeds are like? (Ones which don't use the shell's file copy code, of course. The DOS Copy command is a quick way to test that, or a tool like TeraCopy.)
You can't disable Opus's copy routines, but you can configure Opus to do something else instead, in most cases. Commands for copying via the shell can be found here and via TeraCopy can be found here.