windowserver activity monitor tip
I also wondered this as it was using up nearly 50 CPU in activity monitor
I was burning an idvd at the time which shows a live preview of the rendering process in a small video. I tried closing all my windows but idvd to see if it would be less windowserver load on my system and it did not change anything.
What made my window server drop from 50 CPU usage to 4 CPU usage was moving the idvd render window so the small section that displayed video was not visible to the gui. It instantly went down, and when i move the window up so you can see the video preview it instantly went way up. With the preview window not visible, idvd was able to jump it's cpu usage from 100 to 150 on my dual G5, making the render much faster.
So if you want faster renders close the video preview menus of any app, or make them not visible on your desktop.