@Philip7:
Would it work to take a NUC and add a StarTech USB Gigabit NIC (chipset ASIX AX88179) to create a fast OpenVPN pfSense box?
My Zotac CI 323 works fine but the cpu is still the bottleneck when downloading via my VPN provider (17 MBps).
What is the speed of your line? What is your VPN provider?
My mini PC with the same processor of your Zotac CI323 (Celeron N3150), which runs the latest version of pfSense, is able to reach full speed line (100Mbps) connecting to PureVPN or PIA.
It's capable to run snort, pfBlocker and a couple of OpenVPN clients smooth as silk.
Snort is the process that takes more CPU resources under heavy load, while downloading to 100 Mbps the CPU usage barely exceeds 90% if Snort is active, and 40% if Snort is off.
I don't know your needs, but maybe the problem is in the client configuration or in your VPN provider.
Here something about the OpenVPN performance:
https://forum.pfsense.org/index.php?topic=115673.0