So then it doesn’t work across the ubiquity switch just to double check? If so, you will need to enable jumbo frames on that for sure and it is not enabled by default and that could also explain the throughput as it is having to fragment then defragment the frames to cross the switch or iperf is using MSS to determine that it can only send 1500 byte frames, your slower speed is about line rate for 1500 byte frames no matter the speed of the actual link.
ETA: you can verify this by pinging with a large size and setting the “do not fragment” flag, so something like ‘ping -s 2000 -M do ip.addr ’ on Linux, windows uses different flags.