I love eye candy. It puts stress on my system, and makes games run slower, but you have to admit that nothing else can compete.
It shouldn't be doing that. The idea of having hardware accelerated compositing is to offload some of the burden on your CPU to the GPU. I find that as long as I have RenderAccel enabled in my video driver and vsync enabled in compiz, I can watch unaccelerated video (f**k Adobe for not enabling hardware accelerated flash for my card) in full-screen without a hint of choppiness, which is something I could never do without compositing enabled.