Buffer sizes are the main thing, and there's no guaranteed size that'll work well with all setups, so you have to experiment.
Antivirus can also affect things and may treat some software differently to others.
It's also possible that the hardware/drivers/etc. involved only run at full speed when things are done exactly the way File Explorer (via the limited CopyFileEx API) does them. At the moment the only workaround for that is to use something like Copy files via the shell (Windows Explorer) from a button.
In the future we will be making Opus copy using the same CopyFileEx method File Explorer uses by default, when it can, which should solve most issues with setups that don't work well with anything else. (At least when Opus is copying. Almost literally everything else in all software that isn't doing a simple file copy via CopyFileEx will also be slow here, e.g. loading/saving files via apps with network drives involved. Most of those things just don't report speeds so you don't notice. Microsoft optimised CopyFileEx rather than fix the lower level, underlying I/O code which everything ultimately goes through.)