The newly released Ubuntu 9.04 is fantastic in its functionality. But there are some bugs with the new X.org on ATI Radeon graphic cards. Because of this bug, any application refreshes very slowly. If you scroll in firefox, you'll experience very slow and laggy scrolling. To rectify this bug, you have to add the following line in the devices section in the xorg.conf file.
Option "MigrationHeuristic" "greedy"
This will rectify all the problems and now Juanty will start working at its best.
PS: I thank the author of this page for providing the solution to the Xorg problem