If you'd care at all to read though the posts on that site, you will find the explanation for 'low performance'
1) On Tiger, the preference panes belong to the same process space. On Yosemite, they have been isolated into their own processes and need to be loaded when the user switches the pane. Essentially, clicking on the icon in Preferences loads a whole new app. Obviously this will be slower. But it gives you better security and stability. And this has nothing to do with the UI performance whatsoever.
2) Yosemite uses a different animation effect which has the optical illusion of being 'laggy', especially in combination with the pane loading delay.
3) None of this has anything to do with performance. UI performance is about how fast the OS can react to application drawing requests. If there is no drawing request (when the pane is not loaded), talking about performance is useless.
But of course, taking an isolated, misinterpreted case as a proof for your point is much more fun than trying to think and analyse the information, isn't it? Easier as well.
P.S. If you want to find a case for UI performance regression, try mission control on retina screens. This one is a much better bet
Only a shame that it has been actually substantially improved on Yosemite as opposed to Mavericks.