OK, problem seems to be NFS.
Copying over iSCSI with the new NIC went fine. With NFS the problem arises after 10 min. This time however it starts with appr. 30Mb/s and declines to 12Mb/s. No clue why it falls down to that rate and why it does so after some time.
System load on the FN box is next to none.
Increased the number of NFS servers from 4 to 6 and enabled asynchronous handling.
That worked for 15 minutes. So no real improvement.
Have to dig further into NFS ....