I have been working hard to get my H2+ to performe on the 2.5G network with Microtik and Cisco switches.
On direction always end up with 200Mb or less, when testing between two H2+ devices with iperf3.
I have tested EVERY thing. Downgradet switch software, tried other Cisco switches, tweaked iperf etc. etc.
Two dayes of my live spend with testing and reinstalling.
And the problem is the Realtek driver that Odroid link to in the install guide.
I got my Ubuntu 20.04.1 up at full speed today, and im so happy now.

This post is just a warning to other people, that tries to buy new hardware and devices to fix the issue. It seems to be a driver problem, not the switches.
Best regards,
Brian W Andersen
Result with new 5.9.0RC1 kernel (build in support rtl8125b)
Setup
(H2+ cluster3 10.1.1.103/10.2.2.103) <-> (Microtik CRS305-1G-4S+IN) <-> (H2+ cluster4 10.1.1.104/10.2.2.104)
10.1.1.0/24 = 1G network Cisco 3560-CX
10.2.2.0/24 = 2.5G Microtik CRS305-1G-4S+IN
2.5G on Microtik switch.
bwa@cluster3:~$ iperf -c 10.2.2.104
------------------------------------------------------------
Client connecting to 10.2.2.104, TCP port 5001
TCP window size: 1.35 MByte (default)
------------------------------------------------------------
[ 3] local 10.2.2.103 port 39298 connected with 10.2.2.104 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 2.52 GBytes 2.16 Gbits/sec
bwa@clusteer4:~$ iperf -c 10.2.2.103
------------------------------------------------------------
Client connecting to 10.2.2.103, TCP port 5001
TCP window size: 876 KByte (default)
------------------------------------------------------------
[ 3] local 10.2.2.104 port 32788 connected with 10.2.2.103 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 2.47 GBytes 2.12 Gbits/sec
1G on Cisco switch.
bwa@cluster3:~$ iperf -c 10.1.1.104
------------------------------------------------------------
Client connecting to 10.1.1.104, TCP port 5001
TCP window size: 1.04 MByte (default)
------------------------------------------------------------
[ 3] local 10.1.1.103 port 38058 connected with 10.1.1.104 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 1.09 GBytes 940 Mbits/sec
bwa@clusteer4:~$ iperf -c 10.1.1.103
------------------------------------------------------------
Client connecting to 10.1.1.103, TCP port 5001
TCP window size: 629 KByte (default)
------------------------------------------------------------
[ 3] local 10.1.1.104 port 53154 connected with 10.1.1.103 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 1.09 GBytes 940 Mbits/sec