Nah, modern browsers simply offload this to your GPU. They send the background layer to the GPU once, update the foreground as you scroll, and let the GPU composite them for display. Since this is a screen-space effect, and the fill-rate of a modern GPU is immense, this is pretty cheap overall.
Well a lot of people use Chrome and I immediately noticed a problem with the site when using Chrome, so maybe it's still something to keep in mind until it's improved.
