It could be a multitude of things, ranging from your video card being unable to support the graphics settings that are being used for World of Warcraft, to the game overloading the CPU. Here's a couple things you can try.
1.) Go to http://www.systemrequirementslab.com/cyri/
and check to see your computer passes the minimal and/or recommended settings for World of Warcraft. If it doesn't pass recommended settings, check to see which areas your system does not pass in, and reply back including those areas. If it does not pass the minimal settings, reducing game settings may solve the problem. If not, you'll need to upgrade your computer in order for World of Warcraf to run more smoothly on your machine.
2.) Open Task Manager (CTRL+ALT+DEL --> Task Manager) and click the Performance tab. Keep task manager open, and start up and login into World of Warcraft. Once logged in, ALT+Tab to Task Manager and check to see that the game isn't overloading your CPU or Physical Memory. If they are, you'll need to run the game in a lower resolution, or reduce game settings in order to prevent it from doing so.
Hope this helps!