Good idea regarding the maximum speed of a 100 MB/s setup Jürgen. I think everything is setup as Gigabyte but I'll re-check. I'll probably disconnect the switch because the NAS is connected directly to the router. With the switch disconnected there will only be my desktop PC and the NAS in the network. That may point me in the right direction.
My cncern is that this slowdown has only been noticeable recently and if I'm right about that then the NAS hardware should be perfectly capable of producing higher speeds.
All my cables are either 5E or 6.