If you want a really fast cheap websockets server that scales to 250,000 cc on a single node, I wrote xtomp.tp23.org using nginx as a base.
Its a STOMP protocol server written in non-blocking C. +web sockets.
There is no code required in the nginx part because it implements stomp topics and queues. If you want code, it plugs into a rust based stomp server talking reverse STOMP protocol, so you can code all you like in rust and scale out with n x nginx servers.
It is very very low on cpu use. I wrote it because rabbitmq used 5% cpu while idle. Xtomp uses close to 0% cpu when idle because nginx.
I think what they mean is it uses uWebSockets.js, a Node.js module wrapping a C++ implementation.
https://github.com/uNetworking/uWebSockets.js