In a single page application: When using ajax, the ajax request gets the data and the connection is closed. This allows thousands of users to use the system.
In web sockets, each "page" opens a socket connection (to a namespace) and keeps it live however this has thousands of idle connections, this can't be good for performance?
Also, doing tests on firefox, the ajax connection is 20ms latency while each call in socket.io connection is 200ms.