opnsense and FritzBox 6591 throughput...

[dl3it]

I use the following setup:

Internet (cable 1Gb/s) -> FritzBox 6591 -> Opnsense -> LAN
                                                               -> PC (DMZ)

The FritzBox is connected to a PC and the opnsense router (AMD A10-8800, Intel NIC i350-T2).
Running iperf3 on the PC, I get almost 1Gb/s from the PC to wherever (LAN or Internet), in all directions. Running iperf3 from LAN to Internet, the throughput is only ~200Mb/s.
I played with lots of MTU/MSS settings; no influence on throughput. I don't run any services; it's a fresh setup.
I tried "exposed host" in FB; no change.

Any ideas are happily welcome :-)