The main menu has a framelimit of 35 frames per second.
But when you click on singleplayer and the map selection menu pops up my frames per second go up to ~1300.

The effect is that one thread of the game is trying it's best to get the FPS as high as possible. One thread, because I see in the taskmanager it changes from ~5 percent usage for javaw to 50.

The mesurements were taken with the camerastudio mod.
I wasn't able to check the FPS in the newer versions. I also checked it without a modded version and the cpu usage still explode. I know it went
to at least 120 FPS but I wasn't able to mesure any higher with Fraps.
Maybe they fixed it there, because my processor usage didn't explode like before.

It's not that much of a problem but I think when they added it in the main menu they probably want it in the selection menu too. And saving energy is also a thing.