I am putting this under network because it is NOT related to samba as my tests turned out. So let me begin...
I noticed that my file download from my omv is capped at around halve of what it should be (0.5 Gbit/s).
So naturally I first blamed samba but it turned out that it also happened when I transferred stuff with sftp. So not samba.
Then it was time to blame my raid, although having just upgraded that I highly doubted it, but tried it anyway. Copying stuff inside OMV around from/to my raid (to a nvme) always yielded way more then what the nic is capable off, so another miss.
Then I blamed my windows client only to notice that another client had the same limitations.
Then I blamed the NICs on both side and checked with the switch if they connected there full duplex and 1 Gbit/s (which they do).
Then for whatever reason I did an UPLOAD instead of a DOWNLOAD to my OMV and to my surprise I got the full 1 Gbit/s of what my network can handle.
So now I am very confused and hope anyone here has an educated guess on what is going on here and if there is a way to fix this, so I can get my full 1 Gbit/s speed down from my NAS again.