It's not some magical program that can capture movies while you play WoW at 60FPS. Unless you have a BEAST of a PC, it's going to have some impact on your performance.

Also, go to the Movies tab, change the video to Full-Size and change it to 30FPS. It will capture at 30FPS. If you don't like this, try changing it up or down depending on what you need. I find that turning off sound helps too.