CVar maxFPS

From WoWWiki

This cvar sets the maximum FPS of a wow window without having to turn on VSync.

This cvar defaults to 0 - which means it won't create a maximum fps. However it can be changed to get better computer performance, or limit your FPS if your machine is getting close to overheating.

To cap your FPS at 20FPS:

/console maxfps 20

To un-cap your FPS:

/console maxfps 0