Heavy Stutters on Mario Kart Wii

Hi everybody, I'm pretty much a n00b around here I'm trying to play Mario Kart Wii on my Dolphin and it seems there's something wrong: the game starts smooth at 60fps, then after I choose "1 Player mode" audio heavily stutters (but I still get like 25-30fps), and finally, when the race begins everything is tremendously lagging, playing around 7-10fps.

My PC configuration is:
- CPU: AMD Phenom II X4 945 3.0GHz
- GPU: nVidia GeForce 650 Ti Boost 2Gb
- RAM: 4Gb DDR2
- OS: Windows 8.1 Update 1

Since I can usually play PC games pretty well (Bioshock Infinite or Dishonored, just to mension get easily around 45-50 fps at FullHD max settings) I don't know what's the problem. Any help? (I hope so! :-D)

Thank you in advance, in the pictures you can see my Dolphin configuration (I'm following wiki guidelines)

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