Probably a combination of context switching, memory bandwidth, and interrupt handling. If you're going to use a machine with 208 GB of RAM you may as well buy

Probably a combination of context switching, memory bandwidth, and interrupt handling. If you're going to use a machine with 208 GB of RAM you may as well buy a better network card. All major vendors of low latency adapters (which in this case means handling lots of small messages) have transparent drivers for Linux kernel bypass now. For example see OpenOnload. Then you avoid the kernel with no code changes. Once you remove the kernel from the critical path, you should be concerned with I/O resources, which is already illustrated by the article's note about how they get 5x better performance by using four machines with the same total core count. What that does is buy you four times the I/O resources, like memory channels and PCIe lanes.