I'm not sure if it is Directory Opus or the server I'm connecting to, but here goes.
I use a VPN connection to a server at least once a week for file transfers from my PC. The transfer speeds are quite slow, considering that I'm using a Fibre line to the exchange.
Now I'm wondering if it is due to the server I'm connecting to, or does Directory Opus perform some kind of file error checking, etc?
Opus just calls ReadFile and WriteFile in a loop, essentially.
What are the speeds like in File Explorer?
If Explorer has better speeds then there are some buffer size settings you can adjust which help with some devices that are very sensitive to the size of each write request.
When checking speed in Explorer to a network drive, open Task Manager's graph of network activity to see when the network data really stops and measure the time from there. The progress dialog is not always representative of what is really happening. The height of the network graph is also not important compared to the overall time (one may be lower but more consistent and the other higher but with spikes and dips that average out to the same speed).
Also note that whichever program you test with second may benefit from the data being cached by the first program's use of it, which may speed things up if there is a bottleneck on the reading side.
The influence of antivirus and firewall software is a possible issue as they may treat different files or different programs differently. This may include antivirus on the server side.