Unfortunately, the flash-based Youtube movies ask too much of the iBook hardware. My workaround: Save Youtube movies with a little help from keepvid.com in mp4 format. The video player VLC (OSX 10.3.9 at least for the current version), is able to play these files even on a 300MHZ Blueberry or Tangerine iBook in its original resolution. The option to skip frames should be activated.
If you own a 366MHZ or 466MHZ Clamshell with 8MB graphics try Youtube on OS9. With the Mozilla-based Wamcom browser and the appropriate Flash Player plug-in (you have to place it manually in the wamcom plug-in directory) you are able to play them directly - the design and performance is still not perfect.