I had pfSense running on an old Core 2 Duo machine from around 2010 when I worked in MSP. You can run it on just about anything.

The only trouble I had was when I switched to gigabit+ service and had snort running. Snort is single-threaded and that CPU just could not keep up. Suricata would be a better choice given it's natively multi-threaded, but the real limitation there was my setup and not pfSense.