Networking is actually a "right" problem, and should be nearly embarrassingly parallel since two cores and process two unrelated packets at the same time. But, if you look at network stacks on open-source projects, you see a lot of fail. That's the point of my post, as a reference to point to why that spinlock in your networking code is a bad idea, and why it's probably better to replace it with an atomic operation or a lock-free alternative.

For the other problems, you're limited by Amdahl.