There exists a bug in the FreeBSD VirtIO network drivers that massively degrades network throughput on a pfSense server. VirtIO is the interface of choice for Proxmox users and this problem can become troublesome.
The solution is to disable Hardware Checksum Offloading in pfSense. This is in System -> Advanced -> Networking tab. Tick the Disable hardware checksum offload box. You now need to reboot pfSense for this to take effect.