The menus of Mario Kart Wii are extremely demanding due to all the videos they're playing in the background. It's normal if you have a weaker PC for them to lag a lot.
Also, it may be shaders compiling in the background. Mario Kart Wii has some massive ones. You can turn on precompile shaders if you don't want them to compile while teh game is running.
Also, it may be shaders compiling in the background. Mario Kart Wii has some massive ones. You can turn on precompile shaders if you don't want them to compile while teh game is running.