I think I understand why this is true for plain IP forwarding. There isn’t much to break the cache and the lookups are few and fast.

What’s the cheapest (new) computer that can drive a 1Gb port with NAT? With a busy encrypted (wireguard?) connection?

[I don’t think qos has a lot of use in the domestic environment; sure, someone here does it but I think it’s much less mainstream than the features I already mentioned. ]

Such a device could drive my home. But in a couple of years I suspect I’ll want 2Gb or 10.

In the past I’ve tended to use a device until its crappy power supply failed. So I guess I’m hoping for a >5 year life span/upgrade capacity.

For all I know the answer to my question is one of those passively cooled four port n100 bricks from AliExpress. Anecdata happily accepted.

Except actual routers don't handle the traffic on the CPU, they have dedicated hardware to actually handle the packets. The CPU basically runs the OS, configures the hardware router, and does housekeeping tasks (e.g. ARP or FDB expirations, NAT cleanup, etc). The only packets that ever reach it are "trap to CPU" situations that don't require acceleration as those are rare or expensive to implement in hardware (e.g. better suited to a CPU). Those usually include management protocols (ICMP, ARP, NDP, STP, etc) or packets with unknown destination (e.g. the first packet to an IP that requires ARP resolution).

That's how you can have multi-Gbps on a router with a 200MHz MIPS CPU. Or Tbps on a router with a quad-core Xeon.

I assume the real router OS is extremely neutered to basically only route traffic and filter inbound with everything else being removed? But yeah I can definitely see that.