Here's a little trick, hope it's helpful.
Basically don't install flash and download
THIS plugin for safari. Not only youtube, but many other sites (included the one you are watching) allows to play the videos on HTML5.
Needless to say, HTML5 videos play smoothly on a mac. Nothing to do with Flash which more often than not turn a mac into a whirlwind of heat.
If there is an absolute need to use flash on a site. Then you can always use Chrome, which has a built in flash player. Hence there is no need to install flash or whatsoever.