Async operations and Flash Player interferance with JS

Question : Does ExternalInterface use async communications or not? I have read conflicting reports. The docs…strangley enough do not touch on this.

Also - I am wondering about how URLLoader (And MovieClipLoader) works - sync or async? I have noticed that during heavy load operations…(think loading images into a gallery, etc…) that the browser will stop processing Javascript completely…until the swf is done processing.

You cant even roll over image buttons or navigate away from the page when the swf has control. Anyone else have this problem or have any insight into why this is?