Copy speed is a very complex issue and we cannot tell you why without being able to examine your system and repeat the tests. It can be influenced by antivirus, firewall, caching, buffer size, drivers, network hardware, NAS firmware, differences in how things report speed (and when the operation finishes), and more.
(Do not rule out NAS firmware. Several times now we've seen devices only initially tested with Explorer which run very slowly if anything copies to them even slightly differently, which were fixed by firmware updates.)
Within Opus itself, this setting can influence copy speed in some situations (usually when some device or low-level software in the chain has pathological issues with certain buffer sizes):
Preferences / Miscellaneous / Advanced: copy_buffer_size
This setting can also be important, although it should not affect writing to network folders (only how fast data is read or written on the local side; that could still be a factor, of course, since you are reading from a local drive):
Preferences / Miscellaneous / Advanced: copy_nonbufferio_threshold
Apart from that, Opus just reads data in a loop on one thread, and writes data in a loop on another thread, as fast as the underlying components will process the requests, meaning the other factors are external to Opus.