Have they fixed the ridiculous "Only one connection allowed at a time to an http server" change yet? :/
As far as I'm aware, the browser is fairly unusable because of this.
There is however a bug with long-polling connections: If you fire off the XHR for the long-polling endpoint (or Server Site Events endpoint for that matter) before window.onload has fired, then Safari will not fire onload (nor load any other external resource) until the long-polling connection closes or returns something.
What you are seeing is people not updating their scripts yet to take this bug into account.
Why? Shouldn't the expectation be that the bug gets fixed?