Earlier quoted context omitted.
A CPU from the last 20 years can route traffic at gigabit speed. It's only something to worry about for a Raspberry Pi3 or something similarly 'crippled'.
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.…
What's the cheapest new computer you can find? That will work. If you have PPPoE, you need to be a bit more careful; depending on your OS and NICs, it's possible for inbound traffic to only use one core; low power laptop cpu may not have enough throughput from a single cpu, but my information is a little dated.
I did 1G NAT on a dual core haswell [1] for a long time.
[1] https://www.intel.com/content/www/us/en/products/sku/82723/i...