First, I’d look at what CHR virtual machine network settings, specifically the emulated network card on Hyper-V – that should not be that slow if this was all done on same hardware.
But since X86 already works on same hardware, using PCI passthrough for the network card may possible and/or good idea if you’re looking to maximize performance. PCI passthrough requires multiple ethernet ports since once passthrough, you couldn’t get to the Proxmox virtual server if all ethernet ported are passthough’ed. Now without passthrough, all the traffic would hit the Linux/Proxmox via a software bridge – likely quick stop, but won’t happen on X86 version, maybe the difference in raw speed. A quick forum search turns up… so a little unclear what to expect with Proxmox:
http://forum.mikrotik.com/t/maximizing-proxmox-chr-performance/148930/7
…now whether CHR includes same network drivers as X86, hard to know without test it.
We normally use VMWare with CHR – never an issue. I believe the hypervisor is free as long as you don’t want the management features – but you can read the exact rules. With ESXi, we use the VMXNET3 network and in most cases for CHR enable Promiscuous Mode on virtual switch too. We don’t use passthrough since the CHR in our case is the router for other VM guests on same box.