This is disabled by default in the pfSense 2.3.4 image that we provide.
TCP traffic passing through a pfSense VM (with OpenVPN for example) is not stable (Connection timeout for example) but pings are working.
Checksum offloading is not supported for virtualized pfSense. More information is available in the pfSense documentation.
You need to disable checksum offloading on your pfSense VM:
Check the Disable hardware checksum offload box under System > Advanced on the Networking tab and manually reboot pfSense after saving the setting as shown below.