Are you running WAN and LAN on the same subnet connected to the same gateway?? That's going to cause all kinds of weird issues. You can change LAN to a unique subnet instead.
i5-6500T can handle way more than gigabit throughput.
You can still do the setup nearly the same, but you'll need to change the opnsense LAN to its own unique subnet, and put PC2 on that subnet with a manual IP setup.
Make sure you don't enable DHCP on opnsense too, since that will really interfere with the existing router.
The only real downside to this, is PC2 traffic to access PC1 or other devices on the main router, will have to go through opnsense and you'll need firewall rules for that.