OpenVPN low performance. Do I have MTU problems? Dumps inside

For starters your 'normal' outside tunnel iperf run should be UDP/1194 as the flow on which you have the problem and not TCP/5201. Try with -b 100M first but keep in mind this will produce max size datagrams which is not representative of your VPN traffic (datagram size should be sorta random). Tune with -l option for datagram size and check the results. If the results are not good (I'd say >15 or 20% loss) you may suspect an overloaded Internet router which is dropping your (probably marked best-effort) packets.

Also, it could be interesting to see what performance you get if you switch your VPN tunnel either to an EF Class UDP port (I'd say 5061 because of RTP, but not really sure that all internet routers have correctly configured QoS) or any TCP port.

To me, there's nothing wrong with your setup and your diagnostics don't show anything strange. Also, try another version of OpenVPN or other VPN software.