PENTIUM D SMITHFIELD
How would pfSense run on a PENTIUM D SMITHFIELD with a small LAN, ~ 5/2 WAN? Would OpenVPN be usable?
I guess it will not have any problem to manage a small LAN.
About OpenVPN my old laptop with a Core2 Solo U3500 CPU was able to connect to a VPN provider at 30Mbs.
As you can see from the comparison
a pentium D is a little more fast so I don't think it will be worse of that.
Just to get an idea, here you can find the OpenVPN performance of some CPU used with pfSense: