The reason that the Silverlight viewer has a percentage loading screen is because when you visit the page with the Silverlight viewer in it, it downloads the latest version of the viewer, just like any other part of the page.
By contrast, the deluxe/default Photosynth viewer does not need a loading screen because it has been 100% downloaded and is sitting in your internet browser's plugin folder. It loads almost instantly.
As for a "synth file tak[ing] over 3 or 4 minutes to download and currently... display[ing] only blackness", the viewer only downloads the data for the current view. That includes the pixels neccesary for the photo currently in focus at the current resolution on screen plus low resolution versions of the surrounding photos. After you move to the next photo, all previously downloaded data is dumped as the information for the new view is downloaded. This is so that you do not wind up with a mountain of data in your temp folder.