My keyboard would wear out if I listed everything wrong with Socket.IO. Simply put: it is one of the worst pieces of software I have ever stumbled upon. And the ratio hype-to-quality is off the charts. It's success lies in the fact that most users don't check their numbers and simply follow what everyone else is doing. Combined with the marketing of being "the fastest" which logically means "there exists no such solution in any universe, that outperforms it".

I would recommend WebSocket++ or libwebsockets (if you are not going with µWebSockets).