One of the 1st things I noticed after upgrading to AIGLX with the new ATI drivers was that window resizing was incredibly slow. A quick search on Google yielded a LOT of results for the same problem. The first thing I noticed however, was that they were OLD forum threads. They did mention however, that when resizing a window, the window would be repainted to video memory each step of the way, and that the application would still repaint the window when you were done.
I decided to go look at the settings for the resize plugin with the Settings Manager instead. I found there are different modes for rendering the resize. My plugin was set to the "Normal" mode. I'm not sure if the normal mode has anything to do with the repeatedly re-render problem people had in the past, but I decided to try out some of the other modes. To my pleasant surprise, every other mode of resizing the window is quick and efficient. You can even choose the mode that visually suites you the best.
Renders a rectangle that represents the new size of the window while resizing. The window is resized after you stop dragging. While resizing, the window is still visible in the background at the original size.
Same as the rectangle mode but there is an outline instead of a filled rectangle.
This one is my personal favorite. The window is stretched to the size of the dragging rectangle and doesn't actually repaint until you are finished dragging.