Boy did the Internet became a resource hog.
Only three years back you could kinda watch lowest quality youtube on a Pentium II 200 MHz with Firefox 2.x flash 7.xx using PULP 0.1.
Nowadays we are at Flash 10.3 (actually we are at version 11., Firefox 18.0 and a Pentium III 1000 MHz is chocking on 240p Videos under Lubuntu 12.04, they are hardly watchable. Forget fullscreen mode. WebM (html 5 video), the new kid on the block is such a resource hog that I get a framerate of about one every 1,5 seconds. This is not funny. Even if Lubuntu uses almost 10 times more RAM to show the desktop than PULP 0.1, it still is nowhere near of maxing this computers memory out, which is at 512 MB RAM.
The bottleneck seems to be the flashplayer itself. The free alternatives like Gnash are even slower. so you got to look elsewhere. A quick Internet search came up with Linterna Magica, you can get it there: http://linterna-magica.nongnu.org/
Linterna is a userscript that replaces the flashplayer with you have as a video plugin, like e.g. gecko media player (mplayer based), vlc, totem or xine. All those video players are much more resource friendly than the flash plugin. Follow the extensive installation guide on their webpage and you are good to go. The end result is that you will be able to watch youtube with 360p video fullscreen on a P III 1000 MHz smoothly. Mission accomplished…