First of all, awesome library! This is just what I needed!
My "Space Invaders" like game runs exactly the same - no lag whatsoever! (not that it was all that graphically intense)
Here are the items I had to take care of:
*Bmp would not load. - Had to convert to PNG files using "The Gimp"
*Wav files would not load - Had to convert to mp3 using "FFMPEG"
*Had to reread directions(my bad) and make sure all my images and sounds were set to content in the properties window for each one.
*Was using RectangleF from System.Drawing for collision detection which is unavailble in Silverlight. Switched to SilverSprite.Rectangle works fine for my needs.
*Once those were fixed game ran immediately but the my game was obscured. only part of the play field showed (its 1000x400). I started messing around with width and height in the xaml but that wasn't it. Finally figured out based on the "snake
game demo" that I needed to set graphics.PreferredBackBufferWidth graphics.PreferredBackBufferHeight to my game field size - that was it!
That was really it! Amazing library. Thanks a bunch!