The performance problem in all the Java Swing applications is caused by the Ubuntu specific bug. You can find more information in another bug description.

One known workaround until it's fixed in Ubuntu is:

Under Device section of the /etc/X11/xorg.conf file add the following line:

Option "AccelMethod" "XAA"

You can also add the following option, but some users reported that it makes menu drawing slower:

Option "XAANoOffscreenPixmaps" "true"

While this option should fix the editor scrolling performance, there could be other issues, like slow menu drawing speed. Changing Look&Feel to GTK+ is known to make it much faster.

UPDATE: this bug is fixed in JDK 1.6.0_12, running IntelliJ IDEA under this or later JDK version will also fix the performance problem without the need to edit xorg.conf.