The Window::setAlpha method will trigger texture invalidation and re-cache (also on all of it's children) even when setting an alpha value that is not different to the current one.
Steps to reproduce: Call setAlpha(1.0f) on a layout every frame. Notice that every all contained autorenderingsurfaces are invalidated and re-cached each frame.
Additional information: I have a fix for this and I will attempt a pull request.
OS: All, OS build: All, Platform: CEGUI 0.8