The problem? If you have Kubuntu window compositing turned on and you have an ATI graphics card, resizing a window lags by a few seconds. In effect it means that you drag a window border to resize it, and while your mouse cursor moves to the new destination it takes a few seconds for the window to actually move to the new resized location. This also affected maximising a window from the task bar and even getting the Kmenu to display.
The cause? After a bit of reading, it seems to have something to do with a "fix" added to the Xorg server (one of the components of Kubuntu that handles creating the GUI display) to prevent "garbage" graphical anomalies that displayed for certain Intel graphics cards. The side-effect was that unfortunately, ATI's driver support being what it is (read abysmal), this fix caused resize effects to become incredibly laggy.
The solution? Well, if only there was some way to remove that fix from the Xorg server so that it doesn't affect ATI users (we don't need it after all)? The good news is that you can and it really is not that difficult. We need to replace the existing Xorg server packages with one that does not have this fix in it. Thankfully there is a handy repository that contains just that replacement. Below are the steps to replace the Xorg server with the "unfixed" version that should totally remove the resize issue:
1. Use Alt + F2 to open the run dialog, and type in konsole.
2. Press enter to open the Konsole terminal application.
3. To add the new location for these replacement Xorg files we need to paste the following into the terminal:
sudo add-apt-repository ppa:launchpad-weyland/xserver-nobackfill
4. Once it completes type in:
sudo aptitude update
5. This will reload all the available packages you can install by adding those packages from the new repository we added above. Now type in:
sudo aptitude upgrade
6. You should now get a list of upgrades that will be installed, amongst them being x-server upgrades. Enter Y to prompt asking if you are sure and the downloads will begin.
7. Once things are download and installed, close the Konsole and then logout (you don't need to shutdown but you can if you really want). Once back on the login screen press Alt+E, which forces the Xorg server to restart.
8. Log back in and enjoy the compositing effects as they were meant to be enjoyed.