If open is what you are interested in try OPNsense before pfSense.

I'd recommend starting off with keeping your LAN on a dumb pure L2 switch (no management interface or anything) with your box acting as the upstream swouterwall. It's a lot easier to start with that side of things and see how much load it generates there than it is to throw the whole thing at it and troubleshoot what parts are making it run slow.