100% cpu on minimize

i have an unity game with some buttons as input.
Now when its in the foreground it uses about 10% of my cpu when i minimize it it goes up to 50% (completely use one core)

Ist it a bug, is it in the tracker, will it be fixed soon?
or is there a workaround?

go into window then lightning and turn off the precomputed RealTime GI and baked gi :)!