Hi,
I am using OMV even though I am not expert. I noticed that in windows 10 I can download files at a speed of around 5-6Mbyte/s maybe even less. I have tried to use the FTP protocol with the WinSCP client and it is more or less the same.
The PC I use is not recent I think it has a 100Mbit ethernet port maybe 1Gbit but I'm not sure and I don't know how to check it.