I've been running a number of tests with the MediaPlayerSprite. When the video is to be streamed via RTMP FlashPlayer10 Linux debugger hangs for quite a while before drawing anything to the stage. The problem lies in the OSFMplayer/FlashPlayer because if you do not add the player to the stage via addChild you can hear the video almost instantaneously. However once you addChild the Flash Player goes dead for about 15 secs at least before the stage items appear.
In addition if you set the MPS autoplay to false all your onstage assets appear, however once you click a the play button things freeze up before they stream. The workaround here is to wait about 15 secs before clicking the play button then streaming is instant ( on most occasions ). With non streamed media assets appear once the compile is done.
Ubuntu 9.10 64 bit / FlashBuilder
This sounds like a Flash Player (Linux) bug, as the addChild call wouldn't go through any OSMF code.
I did a quick search in the Flash Player bug database, and the only issue that looks similar is this one:
Recommend you file a bug with the Flash Player team.