Comment by Asmod4n

Network IO is heavily NIC queue bound, if your NIC only has one queue it just makes it slower to do any threading workload against it.

On my ryzen 9 it needs around 8 cores to do the same work in a threaded io loop than you can do single threaded. And the mechanism doesnt matter, you could share an fd, use SO_REUSEPORT or just share memory between threads.

Just doing the sharing makes everything extremely slow. One context switch becomes more expensive than just doing it single threaded.