For embedded it’s not about saving cost it’s about saving limited on board power. The lower the power demand of the device, the smaller and the more you can dedicate to other things.

Again, no reason not to do this change. I'm not clear it helps in this case, though? The savings is in low use time. During high use time, it is the same, right?

Practically none of it would be applicable (if using a commercial router). They all use hardware offloading, and traffic seldom touches the CPU. Only "logical" tasks are raised to the CPU, like ARP resolution and the likes (what’s called "trap to cpu").

If you’re doing custom routing with a NUC or a basic Linux box, however, this would gain massive power savings because that box pretty much only does networking.

Only if you're using busy polling. Very little software uses it, because it's only a good fit if you think pegging a CPU to reduce latency responding to packets is a good trade.

Yup, you’re right. I always forget this patch only applies to opted-in polling and not automatic polling for interrupt moderation.

if you run a VPN server, i assume all the traffic will come to CPU to decrypt, right?