Think about what was just said for a second.
With a capped versus uncapped menu, you're potentially rendering the menu hundreds or thousands of times per second uncapped versus just 30 or 60 times per second if capped.
In StarCraft 2, and many other games that had uncapped framerates in the menu, the GPU would hit 100% utilization due to the menu being rendered thousands of times per second. In other words the menu was pushing the GPU harder than any other part of the game when actually playing the game (SC2 tending to be more CPU limited while gaming). This happened regardless of resolution chosen or graphical settings used.
This was eventually addressed in one of the updates for SC2 by capping menu refresh.
Regards,
SB