so you should know that crystal balls are in short supply or in other words little details have been provided to allow for troubleshooting
My apologise. You are right about the fact that I should have started with a clearer picture for you guys. In not doing so, you guessed my level of cluelessness based on what and how I wrote it. So I got what I deserved out of it. 
I'd recommend the post "Ethernet was slower only in one direction on one device"
from Jeff Geerling proving the opposite experience
That is a very interesting read. Although if you compare the writers story with my problem, you will see that it is a little different at key points.
- I changed the cable and nothing changed
- The speed problem is limited to only one computer, but on every NIC you use there.
- Using a different set of computers (with no OMV), I reach the full speed every time in both directions in my network.
- he talks about having speed issues "sometimes", I have it "all the time". The download speed from OMV is capped at around 0.5 Gbit/s (see the edited iperf above)
I just had a crazy idea and did the following. To explain the settings, you have to know that so far I only used a 2nd NIC inside my NAS only for docker and my DMZ for docker containers. To achieve that I use a docker network in maclan mode, so OMV so far had no direct access itself to that NIC (unconfigured enp1s0f0.102).
- configured the 2nd NIC in OMV so it can be used directly with OMV and assigned an ip address.
- started ipferf3 directly on an OMV/Debian command line and did a test resulting in the same bad/capped performance
- created a new docker network with parent NIC DMZ in bridge mode (so the settings of the configured OMV NIC has to be used)
- started a docker container (latest debian image with just network tools and iperf installed) in that network with same bad results (half gbit capped)
- Then I changed the network and started the same docker container on the docker network I use for my DMZ (configured in maclan mode ) and got FULL 1 Gbit/s both way.
These tests even went over my opnsense firewall hardware as it had to cross networks from my windows computer inside my LAN over to the DMZ network.
So if the NIC is used which is configured by OMV/Debian the result is a capped transfer speed in one direction. Using the docker container in a network configured in maclan mode I get the full speed of the NIC in both directions.
In case you are not familiar with docker and/or maclan mode. This doe not use the host as a bridge but simulate the container being its own hardware on the network with its own mac address, completely ignoring anything which is configured on the LAN adapter inside the host. Before I did these test I had it running with nothing configured at all in OMV, meaning the NIC showed as DISABLED in the network GUI of OMV.
I hope I have explained a bit better why it seems to me a debian/omv config issue with the NIC/base system and not something outside of OMV.