I dualboot Lubuntu 13.10 and MorphOS 3.3 on my 1.67GHz PowerBook G4 and it's nice and responsive, I haven't managed to get hardware accelerated 3D working with the radeon xorg driver for the 9700M though. Do you happen to have a working xorg.conf with functioning 3D for video cards that use the radeon driver?
Could you ask your question in the Apple Users area of the Ubuntu forums:
http://ubuntuforums.org/forumdisplay.php?f=328 ?
There are a lot of users with Lubuntu on PowerBook G4.