Could be a bottleneck somewhere on your LAN with an Ethernet link that can only run at 100BASE-T, either due to some Ethernet device not supporting gigabit, or a bad cable?
I believe higher-end home routers will have hardware accelerated routing, whereas a PC with a few network interfaces will always be routing via the CPU, so a PC won’t always perform better. I’m not sure how much of a difference this really makes.
