Apparently, in 8.10 the ubuntu team found that a particular function in the driver caused instability with Intel graphics chips. So, they removed the function. Trouble is that the same function is used to keep the ATI nice and zippy.
As a result, when using Compiz Fusion with 9.04, machines with ATI graphics chips suffer considerably with several seconds delay in minimising, maximising and restoring windows.
Now you CAN have a go at putting a patch to the Xserver in 9.04 but talk on the forum says that it will be back in 9.10.
Personally, I can wait for 14 days.... Cause I don't want to ruin a perfectly running machine...