Comment # 9 on bug 990356 from Neil Brown

Maybe if you run
  systemctl restart nfs-config

it will cause the NFS3_SERVER_SUPPORT setting to take effect.
That should run more often than it does.  The issue is fixed upstream
but it haven't bought the change to opensuse yet.

The cause of the long delays I have found to be upstream kernel commit

  Commit: 4b0ab51db32e ("SUNRPC: xs_sock_mark_closed() does not need to trigger
socket autoclose")

in Linux 4.3.
Reverting that makes the delays go away.  I've submitted this revert for the
tumbleweed kernel (and SLE12-SP2) so it should appear in a kernel update in a
few days.