I would make sure latest driver/FW are installed on the Hypervisor(s).
I would make sure based on the VM’s OS that Mellanox driver are installed.
I would recommend to use iperf2, why?
We recommend using iperf and iperf2 and not iperf3. iperf3 lacks several features found in iperf2, for example multicast tests, bidirectional tests, multi-threading, and official Windows support. For testing our high throughput adapters (100GbE), we recommend to use iperf2 (2.0.x) in Linux and NTTTCP in Windows.
Are you getting any drops/discards or buffer errors (ie: rx_out_of_buffer)?
(VM’s and/or Switch(s) as applicable).
If you reverse client/server on the VM’s, are you observing the same issue?
Do you have flow control configure on the host and switch (if switch applicable).
If switch applicable, did you test b2b accordingly? (no switch).
I am not sure what you meant by “via live cd”.
What is the iperf CLI options you are using on the client/server?