In CocoonJS tests there many demo's with full screen images running at 60fps. However C2 on my device crawls with TileBackground at 256x256 over the screen. There is definitely something wrong.
I will take you up on that offer. Try this
original simple: https://dl.dropbox.com/u/14087254/ballbouncetest.capx
modify resolution and position if needed. try to be sure that when using letterbox your getting exacctly the availble screen size :)