Please note that the original Quake game was running by default in a 320x240 screen resolution and most critical parts were coded in highly optimized assembly code (John Carmack).

The demonstrated resolution here is 640x480 and is written in pure C#. The rendering part is as fast as possible given the current Silverlight limitations. Please note that no real optimizations were performed yet so this should improve soon.
Also we could benefit of dual core systems provided that we tweak the rendering to be multi-threaded.

In any case, on a same test case computer, the original assembly-optimized version is not running that fast with higher resolution. Of course, the OpenGL version outperforms everything with the help of hardware-accelerated 3D.