Antivirus and firewall can incorrectly block WebSocket connections.
Kaspersky PURE web traffic monitor
We've had reports that Kaspersky PURE web traffic monitor can take a long time to scan web traffic and therefore can considerably slow down connections. It is possible to disable the web traffic monitor.
Avast! WebShield (sometimes known as Web Protect)
We have had reports of Avast! Web Shield causing problems with WebSocket connections. This is just one component of the Avast! suite of antivirus components and can be individually disabled.
We have contacted Avast! about this and the information we've been given on this is as follows:
the current avast! Web shield still keeps the original HTTP proxy concept, it expects the request-response protocol in the client-server computing model. It means that it's not compatible with the latest web standards that allows real-time communication between "peers" (i.e. where either the client or the server can send the initial request - it changes the standard client-server architecture to a virtual peer-to-peer network) and message exchange patterns other than request-response. This concept helps to increase the security because the Web Shield limits the inbound communication (on selected HTTP ports) to recognized responses to known requests. This important aspect makes the implementation more complicated in the security programs than in common web browsers. Unfortunately the Web Shield design must be changed to address the problem. The full support for the latest web standards will be probably introduced in avast! 7 (this major program update is planned for the next year).
A related topic on the Avast! forum can be found here: http://forum.avast.com/index.php?topic=68270.0