Linux =/= Linux. If you put the latest vanilla Ubuntu on an old computer and expect it to run well, it won't. Lots of unnecessary daemons, lots of enterprise features and stuff like SELinux which, while nice in theory, won't be necessary for the average user and will slow things down substantially.
I have an old 1200 MHz laptop that I've treated to an Arch Linux install running LXDE. It beats every other distro I've tried to run on it (Ubuntu, Fedora, Mint, Debian, even VectorLinux and Crunchbang). Usually 240p YouTube is a no-go, but with Arch I can watch 360p and it's smooth. XVid is also no problem. H264 needs beefier hardware.
You do not learn how a Linux works by using GUI configuration tools. That's why it's been said that "Linux is ****, stuff breaks and then you may have to get down to the command line". This is true, but you were supposed to START at the command line. Editing a .conf file is so elegant, I can't believe I didn't see it like this before. Addiction has set in.