In my testing of Lubuntu and Ubuntu-MATE on my iBook G4 one things that I notice often is that when playing back video using mpv my overall CPU usage spike to the high 90s. The fan on my machine start spinning really loud and fast. I thought I would do a comparison of with Debian to see how they compare. Here are some of my findings.
Machine
iBook G4
1.42 GHz CPU
1.5 GB RAM
These findings are based on lxtask
LXDE and Openbox
Overal CPU average 50%-60%
MPV process 30%-40%
RAM 140MB
The fans not once came one and there only brief spikes to 80%-90% CPU usage.
XFCE
Overal CPU average 70%-80%
MPV process 30%-40%
RAM 240MB
Even with the higher numbers due to XFCE being a heavier desktop than LXDE, video playback did not cause my fans to run. As much as I love Ubuntu based distros Debian is by far the better performer. I believe a lot of it is due how I set up Debian. When I installed the system I manually chose what applications and services were installed on my machine. Ubuntu on the other hand chooses for you what comes with the desktop. The advantage to this is you get a compete working system pretty easily. The cost of this is have a performance hit. On newer machines this may not be a big deal but on older G3 and G4 PPC machines it is a great deal. You get no more than 2GB of RAM on some machines, and a single processor for the iBooks and PowerBooks. I would never say stop using Lubuntu or Ubuntu-MATE, however give Debian a try. Yes it will take some more work than Ubuntu, however the benefits of running it are great.