We use uWebSockets as the main WebSocket server engine for SocketCluster http://socketcluster.io/ - We liked it from the beginning because it was a drop-in replacement for the 'ws' Node.js module and performed MUCH better. Even in the worst case, we found it to be more than 2x faster than ws.

Alex (the author) has been very thorough and made sure that it passes all the popular WebSocket standard tests so the switch has been seamless.