Not knowing exactly which plugin is causing it, it would be difficult to fix. You should start removing all but one and proceed to test which one is the most troublesome.
I use none of those and not knowing specifically their inner workings it could be a bunch of things such as trying to access outbound connections and not being able to as you suggested. But the plugin should return an error and not hang on it without a response. So its either lack of proper programming or a bug in your system.