In the current beta, Silverlight doesn't support fast start or advanced fast start. But looking at your description, I don't know if that will help you at all since fast start doesn't change the number of seconds that the video is buffered, it only allows the video to start more quickly. If you want to get closer to real-time streams, we do provide a BufferingTime property on the MediaElement where you could decrease the amount of buffer that we keep. This should allow you to be much closer to real-time but you will be more succeptible to network hickups and congestion issues.