I have 2 ESX with proxmox and one VM with openmediavault. When i want to copie a file betwwen this 2 VM i have a speed from 150MB/s.
I have a 10Gb card ethernet on my 2 esx.
Did you do iperf benchmarks to be sure that no something like storage is the bottleneck? iperf will read/write to RAM when testing network latency/throughput.