it's a geforce3 card with 64mb. the higher the window resolution the slower the framerate instead of an increase of the cpu usage.
The screen resolution shouldn't affect the CPU usage at all. The higher the resolution, the more the graphic card's GPU has to compute and therefore the framerate will sink.
Does this "low framerate" only apply to the Jogl demos or to other native tasks, too? You could cross-check several of the Jogl-demos by running their native equivalent. The red devil knight (Shadow Volumes) for example is located here: http://www.nvidia.com/view.asp?IO=inf_shadow_volumes