OK. So I did those tests, with and without the -R option, between two machines, one running Ubuntu 17.10, and one running Ubuntu 18.04, and I got bandwidth results hovering around 935 Mbits/sec both ways, regardless of which was the serving machine for the iperf3 test.
As I understand it, iperf3 is a raw bandwidth test, unrelated to plex. One would also use it with one’s router and everything turned off, to test ISP performance, for example.
So the results you got for 16.04 vs 18.04 would seem to indicate to me, based on my own results, that when you did the test on 18.04, some driver or something else was interfering with the network speed.
At least for me, on my hardware, with these tests, it would suggest that if I upgrade the 17.10 plex server to 18.04, I should expect to continue to see bandwidth results in the 935Mbits/sec range.