Comparing the SceneGraph and Canvas versions of BitmapTest w/ 600 monsters at fullspeed, SceneGraph shows a big speed advantage over canvas:

Canvas: 110 fps
SG: 330 fps

This is primarily due to the call to cv.save() in NGCanvas.renderContent(). Without it, we see a big performance boost:

Canvas w/o cv.save(): 350 fps

cv.save() is there to save the contents of the canvas so we can guarantee persistence when the screen is locked and the HW buffers are lost. So we're reading the pixels back on every frame.

Once we move to DX10, there is a mechanism where they can save the buffers for us only when the surfaces are lost.