Computer ran Mists but won't run Wrath?

I downloaded the Mists of Pandaria expansion on to my computer when I played it some time ago; it's been a while, but I decided to switch to Icecrown/Wrath of the Lich King, and when I open it, it lags HORRIBLY. Even with all the graphics turned down to their lowest setting, the game is completely unplayable. I'm only confused because, with Mists, it ran perfectly? Is this because of the potentially newer programming in Mists that makes it run better on lower-end systems, as WoW has always been working to perfect? Is it because the population is higher? Or is it just my computer finally giving up on the game?

Anything would be helpful at this point,

Thanks so much !!
Lana

delete that Wotlk client and download "Wotlk (old)" from here: https://www.warmane.com/account