I almost gave up on this, but luckily I found a good solution that works for me. I own a Powerbook G4 1Ghz 1.25Gb RAM. I noticed that altough movie playback is great in VLC when it comes down to the flash player it has horrible performance. In fullscreen mode even in 360p playback youtube stutters and hangs and does a performance of around 8-12 fps on my system.
Here is a step by step description of what I did. This is for Firefox so make sure you install it!
What we are trying to do is to change the flash video player for Youtube only. Forcing an alternative player that is better supported by the OS. This can be either VLC or Quicktime since both players support FLV and MP4 playback.
Step 1: Get Greasemonkey
In firefox you go to Tools->Add-ons->Get add-ons
Search for the word “greasemonkey” and click “Add to firefox”. After installing this will restart your firefox.
Step 2: Go to userscript.org and get “Youtube without flash auto” script
Click “install”. A window will popup allowing you to install the script.
Step 3: Go to youtube.com and open a video. The flash player will initialize and will stop. A new menu will appear under every video. Click the “YWOF Prefs” button. A config screen will appear.
Step 4: for default quality choose MP4 and the quiality you would like your vids to default to. I use the quicktime plugin so I chose MP4. For “player” leave “Generic”.
Step 4 (Optional): If you would like to use VLC make sure you download and install VLC and the VLC Internet Plugin. You can find both for MacOS 10.4 here: http://www.videolan.org/vlc/download-macosx.html Again: only do this if you are sure you want to use VLC. I am using QuickTime. No need to do VLC unsless QT doesn’t work for you.
Step 5: Next time you open any youtube video the QuickTime plugin will play it. Double click the screen to go to fullscreen mode and enjoy stutter-free youtube on your Powerbook G4 with MacOS 10.4.* (to exit fullscreen: double-click the video, as keyboard did not work for me)
Hope this helps. Figuring this out surely made my day since the biggest issue with my Powerbook G4 for me was the inability to use youtube.
EDIT: Upon further tweaking I noticed that the VLC plugin work much better for me then QT.